Diffusion Policies as an Expressive Policy Class for Offline Reinforcement Learning (2208.06193v3)

Abstract: Offline reinforcement learning (RL), which aims to learn an optimal policy using a previously collected static dataset, is an important paradigm of RL. Standard RL methods often perform poorly in this regime due to the function approximation errors on out-of-distribution actions. While a variety of regularization methods have been proposed to mitigate this issue, they are often constrained by policy classes with limited expressiveness that can lead to highly suboptimal solutions. In this paper, we propose representing the policy as a diffusion model, a recent class of highly-expressive deep generative models. We introduce Diffusion Q-learning (Diffusion-QL) that utilizes a conditional diffusion model to represent the policy. In our approach, we learn an action-value function and we add a term maximizing action-values into the training loss of the conditional diffusion model, which results in a loss that seeks optimal actions that are near the behavior policy. We show the expressiveness of the diffusion model-based policy, and the coupling of the behavior cloning and policy improvement under the diffusion model both contribute to the outstanding performance of Diffusion-QL. We illustrate the superiority of our method compared to prior works in a simple 2D bandit example with a multimodal behavior policy. We then show that our method can achieve state-of-the-art performance on the majority of the D4RL benchmark tasks.

Analysis of "Diffusion Policies as an Expressive Policy Class for Offline Reinforcement Learning"

This paper introduces the concept of using diffusion models as a policy class in Offline Reinforcement Learning (RL), a framework known as Diffusion Q-learning (Diffusion-QL). The authors address a critical limitation in offline RL: the difficulty in learning optimal policies from static datasets due to errors arising from out-of-distribution actions. The paper pioneers the application of diffusion models in the policy representation, leveraging their expressive potential to capture complex distributions inherent in offline datasets.

Overview and Methodology

Offline RL poses challenges primarily due to the inability to query the environment for new data, making policy learning susceptible to overestimations for unseen actions. Traditional approaches have sought to mitigate this by introducing policy regularizations or employing simplified policy classes, but often at the expense of expressiveness and solution optimality. The authors propose using diffusion models, which are highly-expressive deep generative models capable of representing multivariate and multimodal distributions, in order to embody the policy.

The core contribution of the paper is the development of a novel approach, Diffusion-QL. This method utilizes a conditional diffusion model to directly model the policy. The key innovation lies in incorporating a behavior-cloning term and a Q-learning improvement term directly into the diffusion process, harmonizing policy regularization and policy improvement. This approach allows the model to maintain proximity to the behavior policy while exploring high-value action regions, thus leveraging the expressiveness of diffusion models to overcome previous policy representation constraints effectively.

Empirical Evaluation

The authors provide robust empirical validation of Diffusion-QL across various benchmark tasks in the D4RL suite, demonstrating the superiority of their approach compared to several baseline methods, including TD3+BC, BCQ, and CQL, among others. Strong numerical performance is reported, particularly in scenarios where multi-modal action distributions are prevalent. The proposed model not only improves the state-of-the-art on most tasks but also shows significant promise in challenging environments like AntMaze, where sparse rewards and sub-optimal trajectory stitching are crucial.

Notably, the paper includes a detailed examination of the effect of varying diffusion timesteps, settling on practical ranges that provide a balance between policy expressiveness and computational efficiency.

Implications and Future Directions

The findings of this paper hold substantial implications for both the theoretical understanding and practical applications of RL. The introduction of diffusion models into policy representation opens new avenues for tackling the exploration-exploitation dilemma inherent in RL. The expressiveness of these models paves the way for capturing more nuanced behaviors from offline datasets, which can be particularly beneficial in applications where interactions are limited or costly, such as autonomous driving and healthcare.

In terms of future developments, enhancing the computational efficiency of diffusion-based policies remains a pertinent area of investigation. The paper acknowledges the existing computational bottlenecks related to the iterative nature of diffusion sampling and suggests that future works could explore diffusion model distillation or other techniques to mitigate these issues. Additionally, adapting diffusion policies for online RL environments and further exploring their potential with combinatorial action spaces could be promising directions.

This research reaffirms the evolving capability of generative models in RL, expanding the toolkit available to researchers and practitioners aiming to develop robust, efficient, and accurate RL systems in environments where direct exploration is unfeasible.