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Tian Lan

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34 papers
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34

AAAI Conference 2026 Conference Paper

WikiREVIEW: A Multi-Perspective Review Framework for Automatic Wiki-Style Article Generation

  • Guo-Biao Zhang
  • Zhijing Wu
  • Tian Lan
  • Ding-Yuan Liu
  • Yu-Shi Zhu
  • Xian-Ling Mao

As a knowledge-intensive and challenging task, automatic generation of long-form wiki-style articles has garnered increasing attention from researchers due to its ability to efficiently integrate, organize and present vast amounts of both structured and unstructured knowledge. To the best of our knowledge, most of the existing mainstream state-of-the-art methods for automatic wiki-style article generation typically follow a "one-shot generation" paradigm: given a topic, (1) first generating a structured outline, (2) then independently and in parallel generating the content of each outline chapter in a one-shot using the chapter title and references. However, the core limitation of the paradigm lies in its disregards inter-chapter correlation and lacks post-generation revision and refinement, resulting in content redundancy, weak relevance and logical inconsistency. To address these issues, we propose WikiREVIEW, a novel multi-perspective review framework for automatic wiki-style article generation. Specifically, our proposed method introduces multi-perspective experts to review the content of each outline chapter at both chapter and paragraph levels following the initial generation, offering evaluation feedback and continuously refining the numerous deficiencies in the initial long-form article, ultimately achieving high-quality wiki-style article generation. Extensive experimental results on the public English dataset FreshWiki and our own constructed high-quality Chinese dataset ChineseWiki, demonstrate that our proposed WikiREVIEW significantly outperforms existing state-of-the-art automatic wiki-style article generation methods across all automatic evaluation metrics and human evaluation.

ICLR Conference 2025 Conference Paper

Block-Attention for Efficient Prefilling

  • Dongyang Ma
  • Yan Wang
  • Tian Lan

We introduce Block-attention, an attention mechanism designed to address the increased inference latency and cost in Retrieval-Augmented Generation (RAG) scenarios. Traditional approaches often encode the entire context in an auto-regressive manner. Instead, Block-attention divides retrieved documents into discrete blocks, with each block independently calculating key-value (KV) states except for the final block. In RAG scenarios, by defining each passage as a block, Block-attention enables us to reuse the KV states of passages that have been seen before, thereby significantly reducing the latency and the computation overhead during inference. The implementation of Block-attention involves block segmentation, position re-encoding, and fine-tuning the LLM to adapt to the Block-attention mechanism. Experiments on 11 diverse benchmarks, including RAG, ICL, and general domains, demonstrate that after block fine-tuning, the Block-attention model not only achieves performance comparable to that of full-attention models, but can also seamlessly switch between the block and full attention modes without any performance loss. Notably, Block-attention significantly reduces the time to first token (TTFT) and floating point operations (FLOPs) to a very low level. It only takes 45 ms to output the first token for an input sequence with a total length of 32K. Compared to the full-attention models, the TTFT and corresponding FLOPs are reduced by 98.7\% and 99.8\%, respectively. Additionally, in Appendix A, we elaborate on how Block-attention is applied in Game AI scenario and the substantial potential benefits it entails. We strongly suggest researchers in the gaming field not to overlook this section.

AAAI Conference 2025 Conference Paper

Learning to Collaborate with Unknown Agents in the Absence of Reward

  • Zuyuan Zhang
  • Hanhan Zhou
  • Mahdi Imani
  • Taeyoung Lee
  • Tian Lan

With the advancements of artificial intelligence (AI), emerging scenarios involving close collaboration between AI and other unknown agents are becoming increasingly common. This requires sometimes training AI agents to collaborate with unknown agents in the absence of a reward function -- which may be unavailable to the AI agents or even undefined by the unknown agents themselves -- thus posing news challenges to existing learning algorithms that often require knowing the shared reward. In this paper, we show that effective teaming with unknown agents can be achieved in the absence of a reward function, through actively modeling other unknown agents and reasoning about their latent rewards from available interaction/observation history. In particular, we propose a novel framework that leverages a kernel density Bayesian inverse learning method for active reward/goal inference and prove that multi-agent reinforcement learning guided by the inferred reward signals can converge to an optimal policy teaming with unknown agents. The result enables us to develop an adaptive policy update strategy, through the use of a family of pre-trained, goal-conditioned policies, further eliminating the need for online retraining. The proposed solution is evaluated using a wide range of diverse unknown agents of latent and even non-stationary reward. Our solution significantly increases the teaming performance between AI and unknown agents in the absence of reward.

NeurIPS Conference 2025 Conference Paper

MALinZero: Efficient Low-Dimensional Search for Mastering Complex Multi-Agent Planning

  • Sizhe Tang
  • Jiayu Chen
  • Tian Lan

Monte Carlo Tree Search (MCTS), which leverages Upper Confidence Bound for Trees (UCTs) to balance exploration and exploitation through randomized sampling, is instrumental to solving complex planning problems. However, for multi-agent planning, MCTS is confronted with a large combinatorial action space that often grows exponentially with the number of agents. As a result, the branching factor of MCTS during tree expansion also increases exponentially, making it very difficult to efficiently explore and exploit during tree search. To this end, we propose MALinZero, a new approach to leverage low-dimensional representational structures on joint-action returns and enable efficient MCTS in complex multi-agent planning. Our solution can be viewed as projecting the joint-action returns into the low-dimensional space representable using a contextual linear bandit problem formulation. We solve the contextual linear bandit problem with convex and $\mu$-smooth loss functions -- in order to place more importance on better joint actions and mitigate potential representational limitations -- and derive a linear Upper Confidence Bound applied to trees (LinUCT) to enable novel multi-agent exploration and exploitation in the low-dimensional space. We analyze the regret of MALinZero for low-dimensional reward functions and propose an $(1-\tfrac1e)$-approximation algorithm for the joint action selection by maximizing a sub-modular objective. MALinZero demonstrates state-of-the-art performance on multi-agent benchmarks such as matrix games, SMAC, and SMACv2, outperforming both model-based and model-free multi-agent reinforcement learning baselines with faster learning speed and better performance.

RLC Conference 2025 Conference Paper

Reinforcement Learning for Human-AI Collaboration via Probabilistic Intent Inference

  • Yuxin Lin
  • Seyede Fatemeh Ghoreishi
  • Tian Lan
  • Mahdi Imani

Effective collaboration between humans and AI agents is increasingly essential as autonomous systems take on critical roles in domains like disaster response, healthcare, and robotics. However, achieving robust human-AI collaboration remains challenging due to the uncertainty, complexity, and unpredictability of human behavior, which is often difficult to convey explicitly to AI agents. This paper presents a belief-space reinforcement learning framework that enables AI agents to implicitly and probabilistically infer latent human intentions from behavioral data and integrate this understanding into robust decision-making. Our approach models human behavior at both the action (low) and subtask (high) levels, combining these with human and agent state information to construct a comprehensive belief state for the AI agent. We demonstrate that this belief state follows the Markov property, enabling the derivation of an optimal Bayesian policy under human and task uncertainty. Deep reinforcement learning is used to train an offline Bayesian policy across a wide range of human and task uncertainties, allowing real-time deployment to support effective human-AI collaboration. Numerical experiments demonstrate the effectiveness of the proposed policy in terms of cooperation, adaptability, and robustness.

RLJ Journal 2025 Journal Article

Reinforcement Learning for Human-AI Collaboration via Probabilistic Intent Inference

  • Yuxin Lin
  • Seyede Fatemeh Ghoreishi
  • Tian Lan
  • Mahdi Imani

Effective collaboration between humans and AI agents is increasingly essential as autonomous systems take on critical roles in domains like disaster response, healthcare, and robotics. However, achieving robust human-AI collaboration remains challenging due to the uncertainty, complexity, and unpredictability of human behavior, which is often difficult to convey explicitly to AI agents. This paper presents a belief-space reinforcement learning framework that enables AI agents to implicitly and probabilistically infer latent human intentions from behavioral data and integrate this understanding into robust decision-making. Our approach models human behavior at both the action (low) and subtask (high) levels, combining these with human and agent state information to construct a comprehensive belief state for the AI agent. We demonstrate that this belief state follows the Markov property, enabling the derivation of an optimal Bayesian policy under human and task uncertainty. Deep reinforcement learning is used to train an offline Bayesian policy across a wide range of human and task uncertainties, allowing real-time deployment to support effective human-AI collaboration. Numerical experiments demonstrate the effectiveness of the proposed policy in terms of cooperation, adaptability, and robustness.

AAAI Conference 2025 Conference Paper

ScoreNet: Consistency-driven Framework with Multi-side Information Fusion for Session-based Recommendation

  • Piao Tong
  • Qiao Liu
  • Zhipeng Zhang
  • Yuke Wang
  • Tian Lan

Fusing side information in session-based recommendation is crucial for improving the performance of next-item prediction by providing additional context. Recent methods optimize attention weights by combining item and side information embeddings. However, semantic heterogeneity between item IDs and side information introduces computational noise in attention calculation, leading to inconsistencies in user interest modeling and reducing the accuracy of candidate item scores. These methods also often fail to leverage session-based re-interaction patterns, limiting improvements in score prediction during the decoding phase. To address these challenges, we propose ScoreNet, a consistency-driven framework with multi-side information fusion for session-based recommendation. ScoreNet explicitly models users' persistent preferences, generating consistent decoding scores for candidate items within a unified framework. It incorporates a multi-path re-engagement network to capture re-interaction behavior patterns in a semantic-agnostic manner, enhancing side information fusion while avoiding semantic interference. Additionally, a position-enhanced consistent scoring network redistributes attention scores within sessions, improving prediction accuracy, especially for items with limited interactions. Extensive experiments on three real-world datasets demonstrate that ScoreNet outperforms state-of-the-art models.

AAAI Conference 2025 Conference Paper

Text2Data: Low-Resource Data Generation with Textual Control

  • Shiyu Wang
  • Yihao Feng
  • Tian Lan
  • Ning Yu
  • Yu Bai
  • Ran Xu
  • Huan Wang
  • Caiming Xiong

Natural language serves as a common and straightforward control signal for humans to interact seamlessly with machines. Recognizing the importance of this interface, the machine learning community is investing considerable effort in generating data that is semantically coherent with textual instructions. While strides have been made in text-to-data generation spanning image editing, audio synthesis, video creation, and beyond, low-resource areas characterized by expensive annotations or complex data structures, such as molecules, motion dynamics, and time series, often lack textual labels. This deficiency impedes supervised learning, thereby constraining the application of advanced generative models for text-to-data tasks. In response to these challenges in the low-resource scenario, we propose Text2Data, a novel approach that utilizes unlabeled data to understand the underlying data distribution through an unsupervised diffusion model. Subsequently, it undergoes controllable finetuning via a novel constraint optimization-based learning objective that ensures controllability and effectively counteracts catastrophic forgetting. Comprehensive experiments demonstrate that Text2Data is able to achieve enhanced performance regarding controllability across various modalities, including molecules, motions and time series, when compared to existing baselines.

NeurIPS Conference 2025 Conference Paper

TraffiDent: A Dataset for Understanding the Interplay Between Traffic Dynamics and Incidents

  • Xiaochuan Gou
  • Ziyue Li
  • Tian Lan
  • Junpeng Lin
  • Zhishuai Li
  • Bingyu Zhao
  • Chen Zhang
  • Di Wang

Long-separated research has been conducted on two highly correlated tracks: traffic and incidents. Traffic track witnesses complicating deep learning models, e. g. , to push the prediction a few percent more accurate, and the incident track only studies the incidents alone, e. g. , to infer the incident risk. We, for the first time, spatiotemporally aligned the two tracks in a large-scale region (16, 972 traffic nodes) from year 2022 to 2024: our TraffiDent dataset includes traffic, i. e. , time-series indexes on traffic flow, lane occupancy, and average vehicle speed, and incident, whose records are spatiotemporally aligned with traffic data, with seven different incident classes. Additionally, each node includes detailed physical and policy-level meta-attributes of lanes. Previous datasets typically contain only traffic or incident data in isolation, limiting research to general forecasting tasks. TraffiDent integrates both, enabling detailed analysis of traffic-incident interactions and causal relationships. To demonstrate its broad applicability, we design: (1) post-incident traffic forecasting to quantify the impact of different incidents on traffic indexes; (2) incident classification using traffic indexes to determine the incidents types for precautions measures; (3) global causal analysis among the traffic indexes, meta-attributes, and incidents to give high-level guidance of the interrelations of various factors; (4) local causal analysis within road nodes to examine how different incidents affect the road segments' relations. The dataset is available at https: //xaitraffic. github. io.

IJCAI Conference 2025 Conference Paper

Variational Offline Multi-agent Skill Discovery

  • Jiayu Chen
  • Tian Lan
  • Vaneet Aggarwal

Skills are effective temporal abstractions established for sequential decision making, which enable efficient hierarchical learning for long-horizon tasks and facilitate multi-task learning through their transferability. Despite extensive research, research gaps remain in multi-agent scenarios, particularly for automatically extracting subgroup coordination patterns in a multi-agent task. In this case, we propose two novel auto-encoder schemes: VO-MASD-3D and VO-MASD-Hier, to simultaneously capture subgroup- and temporal-level abstractions and form multi-agent skills, which firstly solves the aforementioned challenge. An essential algorithm component of these schemes is a dynamic grouping function that can automatically detect latent subgroups based on agent interactions in a task. Further, our method can be applied to offline multi-task data, and the discovered subgroup skills can be transferred across relevant tasks without retraining. Empirical evaluations on StarCraft tasks indicate that our approach significantly outperforms existing hierarchical multi-agent reinforcement learning (MARL) methods. Moreover, skills discovered using our method can effectively reduce the learning difficulty in MARL scenarios with delayed and sparse reward signals. The codebase is available at: https: //github. com/LucasCJYSDL/VOMASD.

ICRA Conference 2024 Conference Paper

Aerial Tensile Perching and Disentangling Mechanism for Long-Term Environmental Monitoring

  • Tian Lan
  • Luca Romanello
  • Mirko Kovac
  • Sophie F. Armanini
  • Basaran Bahadir Kocer

Aerial robots show significant potential for forest canopy research and environmental monitoring by providing data collection capabilities at high spatial and temporal resolutions. However, limited flight endurance hinders their application. Inspired by natural perching behaviours, we propose a multi-modal aerial robot system that integrates tensile perching for energy conservation and a suspended actuated pod for data collection. The system consists of a quadrotor drone, a slewing ring mechanism allowing 360° tether rotation, and a streamlined pod with two ducted propellers connected via a tether. Winding and unwinding the tether allows the pod to move within the canopy, and activating the propellers allows the tether to be wrapped around branches for perching or disentangling. We experimentally determined the minimum counterweights required for stable perching under various conditions. Building on this, we devised and evaluated multiple perching and disentangling strategies. Comparisons of perching and disentangling manoeuvres demonstrate energy savings that could be further maximized with the use of the pod or tether winding. These approaches can reduce energy consumption to only 22% and 1. 5%, respectively, compared to a drone disentangling manoeuvre. We also calculated the minimum idle time required by the proposed system after the system perching and motor shut down to save energy on a mission, which is 48. 9% of the operating time. Overall, the integrated system expands the operational capabilities and enhances the energy efficiency of aerial robots for long-term monitoring tasks.

NeurIPS Conference 2024 Conference Paper

APIGen: Automated PIpeline for Generating Verifiable and Diverse Function-Calling Datasets

  • Zuxin Liu
  • Thai Hoang
  • Jianguo Zhang
  • Ming Zhu
  • Tian Lan
  • Shirley Kokane
  • Juntao Tan
  • Weiran Yao

The advancement of function-calling agent models requires diverse, reliable, and high-quality datasets. This paper presents APIGen, an automated data generation pipeline designed to synthesize high-quality datasets for function-calling applications. We leverage APIGen and collect 3, 673 executable APIs across 21 different categories to generate diverse function-calling datasets in a scalable and structured manner. Each data in our dataset is verified through three hierarchical stages: format checking, actual function executions, and semantic verification, improving its reliability and correctness. We demonstrate that models trained with our curated datasets, even with only 7B parameters, can achieve state-of-the-art performance on the Berkeley Function-Calling Benchmark, outperforming multiple GPT-4 models. Moreover, our 1B model achieves exceptional performance, surpassing GPT-3. 5-Turbo and Claude-3 Haiku. We release a dataset containing 60, 000 high-quality entries, aiming to advance the field of function-calling agent domains. The dataset and models are available on the project homepage \url{https: //apigen-pipeline. github. io/}.

AAAI Conference 2024 Conference Paper

ConcaveQ: Non-monotonic Value Function Factorization via Concave Representations in Deep Multi-Agent Reinforcement Learning

  • Huiqun Li
  • Hanhan Zhou
  • Yifei Zou
  • Dongxiao Yu
  • Tian Lan

Value function factorization has achieved great success in multi-agent reinforcement learning by optimizing joint action-value functions through the maximization of factorized per-agent utilities. To ensure Individual-Global-Maximum property, existing works often focus on value factorization using monotonic functions, which are known to result in restricted representation expressiveness. In this paper, we analyze the limitations of monotonic factorization and present ConcaveQ, a novel non-monotonic value function factorization approach that goes beyond monotonic mixing functions and employs neural network representations of concave mixing functions. Leveraging the concave property in factorization, an iterative action selection scheme is developed to obtain optimal joint actions during training. It is used to update agents’ local policy networks, enabling fully decentralized execution. The effectiveness of the proposed ConcaveQ is validated across scenarios involving multi-agent predator-prey environment and StarCraft II micromanagement tasks. Empirical results exhibit significant improvement of ConcaveQ over state-of-the-art multi-agent reinforcement learning approaches.

NeurIPS Conference 2024 Conference Paper

CriticEval: Evaluating Large-scale Language Model as Critic

  • Tian Lan
  • Wenwei Zhang
  • Chen Xu
  • Heyan Huang
  • Dahua Lin
  • Kai Chen
  • Xian-Ling Mao

Critique ability, i. e. , the capability of Large Language Models (LLMs) to identify and rectify flaws in responses, is crucial for their applications in self-improvement and scalable oversight. While numerous studies have been proposed to evaluate critique ability of LLMs, their comprehensiveness and reliability are still limited. To overcome this problem, we introduce CriticEval, a novel benchmark designed to comprehensively and reliably evaluate critique ability of LLMs. Specifically, to ensure the comprehensiveness, CriticEval evaluates critique ability from four dimensions across nine diverse task scenarios. It evaluates both scalar-valued and textual critiques, targeting responses of varying quality. To ensure the reliability, a large number of critiques are annotated to serve as references, enabling GPT-4 to evaluate textual critiques reliably. Extensive evaluations of open-source and closed-source LLMs first validate the reliability of evaluation in CriticEval. Then, experimental results demonstrate the promising potential of open-source LLMs, the effectiveness of critique datasets and several intriguing relationships between the critique ability and some critical factors, including task types, response qualities and critique dimensions.

TMLR Journal 2024 Journal Article

Deep Generative Models for Offline Policy Learning: Tutorial, Survey, and Perspectives on Future Directions

  • Jiayu Chen
  • Bhargav Ganguly
  • Yang Xu
  • Yongsheng Mei
  • Tian Lan
  • Vaneet Aggarwal

Deep generative models (DGMs) have demonstrated great success across various domains, particularly in generating texts and images using models trained from offline data. Similarly, data-driven decision-making also necessitates learning a generator function from the offline data to serve as the policy. Applying DGMs in offline policy learning exhibits great potential, and numerous studies have explored in this direction. However, this field still lacks a comprehensive review and so developments of different branches are relatively independent. In this paper, we provide the first systematic review on the applications of DGMs for offline policy learning. We cover five mainstream DGMs, including Variational Auto-Encoders, Generative Adversarial Networks, Normalizing Flows, Transformers, and Diffusion Models, and their applications in both offline reinforcement learning (offline RL) and imitation learning (IL). Offline RL and IL are two main branches of offline policy learning and are widely-adopted techniques for sequential decision-making. Notably, for each type of DGM-based offline policy learning, we distill its fundamental scheme, categorize related works based on the usage of the DGM, and sort out the development process of algorithms in that field. In addition, we provide in-depth discussions on DGMs and offline policy learning as a summary, based on which we present our perspectives on future research directions. This work offers a hands-on reference for the research progress in DGMs for offline policy learning, and aims to inspire improved DGM-based offline RL or IL algorithms. For convenience, we maintain a paper list on https://github.com/LucasCJYSDL/DGMs-for-Offline-Policy-Learning.

AAMAS Conference 2024 Conference Paper

Projection-Optimal Monotonic Value Function Factorization in Multi-Agent Reinforcement Learning

  • Yongsheng Mei
  • Hanhan Zhou
  • Tian Lan

Value function factorization has emerged as the prevalent method for cooperative multi-agent reinforcement learning under the centralized training and decentralized execution paradigm. Many of these algorithms ensure the coherence between joint and local action selections for decentralized decision-making by factorizing the optimal joint action-value function using a monotonic mixing function of agent utilities. Despite this, utilizing monotonic mixing functions also induces representational limitations, and finding the optimal projection of an unconstrained mixing function onto monotonic function classes remains an open problem. In this paper, we propose QPro, which casts this optimal projection problem for value function factorization as regret minimization over projection weights of different transitions. This optimization problem can be relaxed and solved using the Lagrangian multiplier method to obtain the optimal projection weights in a closed form, where we narrow the gap between optimal and restricted monotonic mixing functions by minimizing the policy regret of expected returns, thereby enhancing the monotonic value function factorization. Our experiments demonstrate the effectiveness of our method, indicating improved performance in environments with non-monotonic value functions.

AAAI Conference 2024 Conference Paper

RGMComm: Return Gap Minimization via Discrete Communications in Multi-Agent Reinforcement Learning

  • Jingdi Chen
  • Tian Lan
  • Carlee Joe-Wong

Communication is crucial for solving cooperative Multi-Agent Reinforcement Learning tasks in partially observable Markov Decision Processes. Existing works often rely on black-box methods to encode local information/features into messages shared with other agents, leading to the generation of continuous messages with high communication overhead and poor interpretability. Prior attempts at discrete communication methods generate one-hot vectors trained as part of agents' actions and use the Gumbel softmax operation for calculating message gradients, which are all heuristic designs that do not provide any quantitative guarantees on the expected return. This paper establishes an upper bound on the return gap between an ideal policy with full observability and an optimal partially observable policy with discrete communication. This result enables us to recast multi-agent communication into a novel online clustering problem over the local observations at each agent, with messages as cluster labels and the upper bound on the return gap as clustering loss. To minimize the return gap, we propose the Return-Gap-Minimization Communication (RGMComm) algorithm, which is a surprisingly simple design of discrete message generation functions and is integrated with reinforcement learning through the utilization of a novel Regularized Information Maximization loss function, which incorporates cosine-distance as the clustering metric. Evaluations show that RGMComm significantly outperforms state-of-the-art multi-agent communication baselines and can achieve nearly optimal returns with few-bit messages that are naturally interpretable.

NeurIPS Conference 2024 Conference Paper

RGMDT: Return-Gap-Minimizing Decision Tree Extraction in Non-Euclidean Metric Space

  • Jingdi Chen
  • Hanhan Zhou
  • Yongsheng Mei
  • Carlee Joe-Wong
  • Gina Adam
  • Nathaniel D. Bastian
  • Tian Lan

Deep Reinforcement Learning (DRL) algorithms have achieved great success in solving many challenging tasks while their black-box nature hinders interpretability and real-world applicability, making it difficult for human experts to interpret and understand DRL policies. Existing works on interpretable reinforcement learning have shown promise in extracting decision tree (DT) based policies from DRL policies with most focus on the single-agent settings while prior attempts to introduce DT policies in multi-agent scenarios mainly focus on heuristic designs which do not provide any quantitative guarantees on the expected return. In this paper, we establish an upper bound on the return gap between the oracle expert policy and an optimal decision tree policy. This enables us to recast the DT extraction problem into a novel non-euclidean clustering problem over the local observation and action values space of each agent, with action values as cluster labels and the upper bound on the return gap as clustering loss. Both the algorithm and the upper bound are extended to multi-agent decentralized DT extractions by an iteratively-grow-DT procedure guided by an action-value function conditioned on the current DTs of other agents. Further, we propose the Return-Gap-Minimization Decision Tree (RGMDT) algorithm, which is a surprisingly simple design and is integrated with reinforcement learning through the utilization of a novel Regularized Information Maximization loss. Evaluations on tasks like D4RL show that RGMDT significantly outperforms heuristic DT-based baselines and can achieve nearly optimal returns under given DT complexity constraints (e. g. , maximum number of DT nodes).

NeurIPS Conference 2023 Conference Paper

A Unified Algorithm Framework for Unsupervised Discovery of Skills based on Determinantal Point Process

  • Jiayu Chen
  • Vaneet Aggarwal
  • Tian Lan

Learning rich skills under the option framework without supervision of external rewards is at the frontier of reinforcement learning research. Existing works mainly fall into two distinctive categories: variational option discovery that maximizes the diversity of the options through a mutual information loss (while ignoring coverage) and Laplacian-based methods that focus on improving the coverage of options by increasing connectivity of the state space (while ignoring diversity). In this paper, we show that diversity and coverage in unsupervised option discovery can indeed be unified under the same mathematical framework. To be specific, we explicitly quantify the diversity and coverage of the learned options through a novel use of Determinantal Point Process (DPP) and optimize these objectives to discover options with both superior diversity and coverage. Our proposed algorithm, ODPP, has undergone extensive evaluation on challenging tasks created with Mujoco and Atari. The results demonstrate that our algorithm outperforms state-of-the-art baselines in both diversity- and coverage-driven categories.

NeurIPS Conference 2023 Conference Paper

Every Parameter Matters: Ensuring the Convergence of Federated Learning with Dynamic Heterogeneous Models Reduction

  • Hanhan Zhou
  • Tian Lan
  • Guru Prasadh Venkataramani
  • Wenbo Ding

Cross-device Federated Learning (FL) faces significant challenges where low-end clients that could potentially make unique contributions are excluded from training large models due to their resource bottlenecks. Recent research efforts have focused on model-heterogeneous FL, by extracting reduced-size models from the global model and applying them to local clients accordingly. Despite the empirical success, general theoretical guarantees of convergence on this method remain an open question. This paper presents a unifying framework for heterogeneous FL algorithms with online model extraction and provides a general convergence analysis for the first time. In particular, we prove that under certain sufficient conditions and for both IID and non-IID data, these algorithms converge to a stationary point of standard FL for general smooth cost functions. Moreover, we introduce the concept of minimum coverage index, together with model reduction noise, which will determine the convergence of heterogeneous federated learning, and therefore we advocate for a holistic approach that considers both factors to enhance the efficiency of heterogeneous federated learning.

AAMAS Conference 2023 Conference Paper

MAC-PO: Multi-Agent Experience Replay via Collective Priority Optimization

  • Yongsheng Mei
  • Hanhan Zhou
  • Tian Lan
  • Guru Venkataramani
  • Peng Wei

Experience replay is crucial for off-policy reinforcement learning (RL) methods. By remembering and reusing the experiences from past different policies, experience replay significantly improves the training efficiency and stability of RL algorithms. Many decisionmaking problems in practice naturally involve multiple agents and require multi-agent reinforcement learning (MARL) under centralized training decentralized execution paradigm. Nevertheless, existing MARL algorithms often adopt standard experience replay where the transitions are uniformly sampled regardless of their importance. Finding prioritized sampling weights that are optimized for MARL experience replay has yet to be explored. To this end, we propose MAC-PO, which formulates optimal prioritized experience replay for multi-agent problems as a regret minimization over the sampling weights of transitions. Such optimization is relaxed and solved using the Lagrangian multiplier approach to obtain the close-form optimal sampling weights. By minimizing the resulting policy regret, we can narrow the gap between the current policy and a nominal optimal policy, thus acquiring an improved prioritization scheme for multi-agent tasks. Our experimental results on Predator-Prey and StarCraft Multi-Agent Challenge environments demonstrate the effectiveness of our method, having a better ability to replay important transitions and outperforming other state-of-the-art baselines.

JMLR Journal 2023 Journal Article

Merlion: End-to-End Machine Learning for Time Series

  • Aadyot Bhatnagar
  • Paul Kassianik
  • Chenghao Liu
  • Tian Lan
  • Wenzhuo Yang
  • Rowan Cassius
  • Doyen Sahoo
  • Devansh Arpit

We introduce Merlion, an open-source machine learning library for time series. It features a unified interface for many commonly used models and datasets for forecasting and anomaly detection on both univariate and multivariate time series, along with standard pre/post-processing layers. It has several modules to improve ease-of-use, including a no-code visual dashboard, anomaly score calibration to improve interpetability, AutoML for hyperparameter tuning and model selection, and model ensembling. Merlion also provides an evaluation framework that simulates the live deployment of a model in production, and a distributed computing backend to run time series models at industrial scale. This library aims to provide engineers and researchers a one-stop solution to rapidly develop models for their specific time series needs and benchmark them across multiple datasets. [abs] [ pdf ][ bib ] [ code ] &copy JMLR 2023. ( edit, beta )

ICRA Conference 2023 Conference Paper

Pedestrian Crossing Action Recognition and Trajectory Prediction with 3D Human Keypoints

  • Jiachen Li
  • Xinwei Shi
  • Feiyu Chen
  • Jonathan Stroud
  • Zhishuai Zhang
  • Tian Lan
  • Junhua Mao
  • Jeonhyung Kang

Accurate understanding and prediction of human behaviors are critical prerequisites for autonomous vehicles, especially in highly dynamic and interactive scenarios such as intersections in dense urban areas. In this work, we aim at identifying crossing pedestrians and predicting their future trajectories. To achieve these goals, we not only need the context information of road geometry and other traffic participants but also need fine-grained information of the human pose, motion and activity, which can be inferred from human keypoints. In this paper, we propose a novel multi-task learning framework for pedestrian crossing action recognition and trajectory pre-diction, which utilizes 3D human keypoints extracted from raw sensor data to capture rich information on human pose and activity. Moreover, we propose to apply two auxiliary tasks and contrastive learning to enable auxiliary supervisions to improve the learned keypoints representation, which further enhances the performance of major tasks. We validate our approach on a large-scale in-house dataset, as well as a public benchmark dataset, and show that our approach achieves state-of-the-art performance on a wide range of evaluation metrics. The effectiveness of each model component is validated in a detailed ablation study.

NeurIPS Conference 2023 Conference Paper

Repetition In Repetition Out: Towards Understanding Neural Text Degeneration from the Data Perspective

  • Huayang Li
  • Tian Lan
  • Zihao Fu
  • Deng Cai
  • Lemao Liu
  • Nigel Collier
  • Taro Watanabe
  • Yixuan Su

There are a number of diverging hypotheses about the neural text degeneration problem, i. e. , generating repetitive and dull loops, which makes this problem both interesting and confusing. In this work, we aim to advance our understanding by presenting a straightforward and fundamental explanation from the data perspective. Our preliminary investigation reveals a strong correlation between the degeneration issue and the presence of repetitions in training data. Subsequent experiments also demonstrate that by selectively dropping out the attention to repetitive words in training data, degeneration can be significantly minimized. Furthermore, our empirical analysis illustrates that prior works addressing the degeneration issue from various standpoints, such as the high-inflow words, the likelihood objective, and the self-reinforcement phenomenon, can be interpreted by one simple explanation. That is, penalizing the repetitions in training data is a common and fundamental factor for their effectiveness. Moreover, our experiments reveal that penalizing the repetitions in training data remains critical even when considering larger model sizes and instruction tuning.

NeurIPS Conference 2022 Conference Paper

A Contrastive Framework for Neural Text Generation

  • Yixuan Su
  • Tian Lan
  • Yan Wang
  • Dani Yogatama
  • Lingpeng Kong
  • Nigel Collier

Text generation is of great importance to many natural language processing applications. However, maximization-based decoding methods (e. g. , beam search) of neural language models often lead to degenerate solutions---the generated text is unnatural and contains undesirable repetitions. Existing approaches introduce stochasticity via sampling or modify training objectives to decrease the probabilities of certain tokens (e. g. , unlikelihood training). However, they often lead to solutions that lack coherence. In this work, we show that an underlying reason for model degeneration is the anisotropic distribution of token representations. We present a contrastive solution: (i) SimCTG, a contrastive training objective to calibrate the model's representation space, and (ii) a decoding method---contrastive search---to encourage diversity while maintaining coherence in the generated text. Extensive experiments and analyses on three benchmarks from two languages demonstrate that our proposed approach outperforms state-of-the-art text generation methods as evaluated by both human and automatic metrics.

TIST Journal 2022 Journal Article

Communication-Efficient Federated Learning with Adaptive Quantization

  • Yuzhu Mao
  • Zihao Zhao
  • Guangfeng Yan
  • Yang Liu
  • Tian Lan
  • Linqi Song
  • Wenbo Ding

Federated learning (FL) has attracted tremendous attentions in recent years due to its privacy-preserving measures and great potential in some distributed but privacy-sensitive applications, such as finance and health. However, high communication overloads for transmitting high-dimensional networks and extra security masks remain a bottleneck of FL. This article proposes a communication-efficient FL framework with an Adaptive Quantized Gradient (AQG), which adaptively adjusts the quantization level based on a local gradient’s update to fully utilize the heterogeneity of local data distribution for reducing unnecessary transmissions. In addition, client dropout issues are taken into account and an Augmented AQG is developed, which could limit the dropout noise with an appropriate amplification mechanism for transmitted gradients. Theoretical analysis and experiment results show that the proposed AQG leads to 18% to 50% of additional transmission reduction as compared with existing popular methods, including Quantized Gradient Descent (QGD) and Lazily Aggregated Quantized (LAQ) gradient-based methods without deteriorating convergence properties. Experiments with heterogenous data distributions corroborate a more significant transmission reduction compared with independent identical data distributions. The proposed AQG is robust to a client dropping rate up to 90% empirically, and the Augmented AQG manages to further improve the FL system’s communication efficiency with the presence of moderate-scale client dropouts commonly seen in practical FL scenarios.

AAMAS Conference 2022 Conference Paper

Multi-agent Covering Option Discovery through Kronecker Product of Factor Graphs

  • Jiayu Chen
  • Jingdi Chen
  • Tian Lan
  • Vaneet Aggarwal

Covering option discovery has been developed to improve the exploration of reinforcement learning in single-agent scenarios with sparse reward signals, through connecting the most distant states in the embedding space provided by the Fiedler vector of the state transition graph. However, these option discovery methods cannot be directly extended to multi-agent scenarios, since the joint state space grows exponentially with the number of agents in the system. Thus, existing researches on adopting options in multi-agent scenarios still rely on single-agent option discovery and fail to directly discover the joint options that can improve the connectivity of the joint state space of agents. In this paper, we show that it is indeed possible to directly compute multi-agent options with collaborative exploratory behaviors among the agents, while still enjoying the ease of decomposition. Our key idea is to approximate the joint state space as a Kronecker graph – the Kronecker product of individual agents’ state transition graphs, based on which we can directly estimate the Fiedler vector of the joint state space using the Laplacian spectrum of individual agents’ transition graphs. This decomposition enables us to efficiently construct multi-agent joint options by encouraging agents to connect the sub-goal joint states which are corresponding to the minimum or maximum values of the estimated joint Fiedler vector. The evaluation based on multi-agent collaborative tasks shows that the proposed algorithm can successfully identify multi-agent options, and significantly outperforms prior works using single-agent options or no options, in terms of both faster exploration and higher cumulative rewards.

AAMAS Conference 2022 Conference Paper

Multi-Objective Reinforcement Learning with Non-Linear Scalarization

  • Mridul Agarwal
  • Vaneet Aggarwal
  • Tian Lan

Multi-Objective Reinforcement Learning (MORL) setup naturally arises in many places where an agent optimizes multiple objectives. We consider the problem of MORL where multiple objectives are combined using a non-linear scalarization. We combine the vector objectives with a concave scalarization function and maximize this scalar objective. To work with the non-linear scalarization, in this paper, we propose a solution using steady-state occupancy measures and long-term average rewards. We show that when the scalarization function is element-wise increasing, the optimal policy for the scalarization is also Pareto optimal. To maximize the scalarized objective, we propose a model-based posterior sampling algorithm. Using a novel Bellman error analysis for infinite horizon MDPs based proof, we show that the proposed algorithm obtains a regret bound of Õ(LKDS p A/T) for K objectives, and L-Lipschitz continous scalarization function for MDP with S states, A actions, and diameter D. Additionally, we propose policy-gradient and actorcritic algorithms for MORL. For the policy gradient actor, we obtain the gradient using chain rule, and we learn different critics for each of the K objectives. Finally, we implement our algorithms on multiple environments including deep-sea treasure, and network scheduling setups to demonstrate that the proposed algorithms can optimize non-linear scalarization of multiple objectives.

NeurIPS Conference 2022 Conference Paper

PAC: Assisted Value Factorization with Counterfactual Predictions in Multi-Agent Reinforcement Learning

  • Hanhan Zhou
  • Tian Lan
  • Vaneet Aggarwal

Multi-agent reinforcement learning (MARL) has witnessed significant progress with the development of value function factorization methods. It allows optimizing a joint action-value function through the maximization of factorized per-agent utilities. In this paper, we show that in partially observable MARL problems, an agent's ordering over its own actions could impose concurrent constraints (across different states) on the representable function class, causing significant estimation errors during training. We tackle this limitation and propose PAC, a new framework leveraging Assistive information generated from Counterfactual Predictions of optimal joint action selection, which enable explicit assistance to value function factorization through a novel counterfactual loss. A variational inference-based information encoding method is developed to collect and encode the counterfactual predictions from an estimated baseline. To enable decentralized execution, we also derive factorized per-agent policies inspired by a maximum-entropy MARL framework. We evaluate the proposed PAC on multi-agent predator-prey and a set of StarCraft II micromanagement tasks. Empirical results demonstrate improved results of PAC over state-of-the-art value-based and policy-based multi-agent reinforcement learning algorithms on all benchmarks.

NeurIPS Conference 2022 Conference Paper

Scalable Multi-agent Covering Option Discovery based on Kronecker Graphs

  • Jiayu Chen
  • Jingdi Chen
  • Tian Lan
  • Vaneet Aggarwal

Covering option discovery has been developed to improve the exploration of RL in single-agent scenarios with sparse reward signals, through connecting the most distant states in the embedding space provided by the Fiedler vector of the state transition graph. Given that joint state space grows exponentially with the number of agents in multi-agent systems, existing researches still relying on single-agent option discovery either become prohibitive or fail to directly discover joint options that improve the connectivity of the joint state space. In this paper, we show how to directly compute multi-agent options with collaborative exploratory behaviors while still enjoying the ease of decomposition. Our key idea is to approximate the joint state space as a Kronecker graph, based on which we can directly estimate its Fiedler vector using the Laplacian spectrum of individual agents' transition graphs. Further, considering that directly computing the Laplacian spectrum is intractable for tasks with infinite-scale state spaces, we further propose a deep learning extension of our method by estimating eigenfunctions through NN-based representation learning techniques. The evaluation on multi-agent tasks built with simulators like Mujoco, shows that the proposed algorithm can successfully identify multi-agent options, and significantly outperforms the state-of-the-art. Codes are available at: https: //github. itap. purdue. edu/Clan-labs/Scalable MAOD via_KP.

JMLR Journal 2022 Journal Article

WarpDrive: Fast End-to-End Deep Multi-Agent Reinforcement Learning on a GPU

  • Tian Lan
  • Sunil Srinivasa
  • Huan Wang
  • Stephan Zheng

WarpDrive is a flexible, lightweight, and easy-to-use open-source framework for end-to-end deep multi-agent reinforcement learning (MARL) on a Graphics Processing Unit (GPU), available at https://github.com/salesforce/warp-drive. It addresses key system bottlenecks when applying MARL to complex environments with high-dimensional state, observation, or action spaces. For example, WarpDrive eliminates data copying between the CPU and GPU and runs thousands of simulations and agents in parallel. It also enables distributed training on multiple GPUs and scales to millions of agents. In all, WarpDrive enables orders-of-magnitude faster MARL compared to common CPU-GPU implementations. For example, WarpDrive yields 2.9 million environment steps/second with 2000 environments and 1000 agents (at least 100× faster than a CPU version) in a 2d-Tag simulation. It is user-friendly: e.g., it provides a lightweight, extendable Python interface and flexible environment wrappers. It is also compatible with PyTorch. In all, WarpDrive offers a platform to significantly accelerate reinforcement learning research and development. [abs] [ pdf ][ bib ] [ code ] &copy JMLR 2022. ( edit, beta )

TCS Journal 2014 Journal Article

On the equality constraints tolerance of Constrained Optimization Problems

  • Chengyong Si
  • Jing An
  • Tian Lan
  • Thomas Ußmüller
  • Lei Wang
  • Qidi Wu

The tolerance value plays an important role when converting equality constraints into inequality constraints in solving Constrained Optimization Problems. Many researchers use a fixed or dynamic setting directly based on trial or experiments without systematic study. As a well-known constraint handling technique, Deb's feasibility-based rule is widely adopted, but it has one drawback as the ranking is not consistent with the actual ranking after introducing the tolerance value. After carefully analyzing how the tolerance value influences the ranking difference, a novel strategy named Ranking Adjustment Strategy (RAS) is proposed, which can be considered as a complement of Deb's feasibility-based rule. The experiment has verified the effectiveness of the proposed strategy. This is the first time to analyze the inner mechanism of the tolerance value for equality constraints systematically, which can give some guide for future research.

NeurIPS Conference 2013 Conference Paper

Latent Maximum Margin Clustering

  • Guang-Tong Zhou
  • Tian Lan
  • Arash Vahdat
  • Greg Mori

We present a maximum margin framework that clusters data using latent variables. Using latent representations enables our framework to model unobserved information embedded in the data. We implement our idea by large margin learning, and develop an alternating descent algorithm to effectively solve the resultant non-convex optimization problem. We instantiate our latent maximum margin clustering framework with tag-based video clustering tasks, where each video is represented by a latent tag model describing the presence or absence of video tags. Experimental results obtained on three standard datasets show that the proposed method outperforms non-latent maximum margin clustering as well as conventional clustering approaches.

NeurIPS Conference 2010 Conference Paper

Beyond Actions: Discriminative Models for Contextual Group Activities

  • Tian Lan
  • Yang Wang
  • Weilong Yang
  • Greg Mori

We propose a discriminative model for recognizing group activities. Our model jointly captures the group activity, the individual person actions, and the interactions among them. Two new types of contextual information, group-person interaction and person-person interaction, are explored in a latent variable framework. Different from most of the previous latent structured models which assume a predefined structure for the hidden layer, e. g. a tree structure, we treat the structure of the hidden layer as a latent variable and implicitly infer it during learning and inference. Our experimental results demonstrate that by inferring this contextual information together with adaptive structures, the proposed model can significantly improve activity recognition performance.

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