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Do Transformer World Models Give Better Policy Gradients? (2402.05290v2)

Published 7 Feb 2024 in cs.LG and cs.AI

Abstract: A natural approach for reinforcement learning is to predict future rewards by unrolling a neural network world model, and to backpropagate through the resulting computational graph to learn a policy. However, this method often becomes impractical for long horizons since typical world models induce hard-to-optimize loss landscapes. Transformers are known to efficiently propagate gradients over long horizons: could they be the solution to this problem? Surprisingly, we show that commonly-used transformer world models produce circuitous gradient paths, which can be detrimental to long-range policy gradients. To tackle this challenge, we propose a class of world models called Actions World Models (AWMs), designed to provide more direct routes for gradient propagation. We integrate such AWMs into a policy gradient framework that underscores the relationship between network architectures and the policy gradient updates they inherently represent. We demonstrate that AWMs can generate optimization landscapes that are easier to navigate even when compared to those from the simulator itself. This property allows transformer AWMs to produce better policies than competitive baselines in realistic long-horizon tasks.

Citations (1)

Summary

  • The paper introduces AWMs that bypass circuitous gradient paths, enabling stable policy gradients over extended time horizons.
  • It provides theoretical bounds demonstrating AWMs achieve polynomial scaling of gradients, unlike the exponential scaling seen in HWMs.
  • Empirical results on tasks like the double-pendulum show AWMs outperform both simulator-based Markovian and traditional model-based approaches.

An Insightful Overview of "Do Transformer World Models Give Better Policy Gradients?"

This paper investigates whether transformer-based world models offer superior policy gradients in reinforcement learning (RL) scenarios, particularly for long-horizon tasks. Adopting transformers in modeling has been prevalent due to their ability to manage dependencies over long sequences, yet their efficacy in policy optimization remains unclear. This paper introduces Actions World Models (AWMs) to address known pitfalls in trajectory unrolling.

Core Analysis and Findings

The authors begin by critiquing existing transformer-based models like History World Models (HWMs), which condition predictions on past states and actions. Such models, although semantically rich, inadvertently form "circuitous gradient paths" during gradient descent. This complexity, propagated through repeated state predictions, poses significant challenges for policy gradient stability in extended time frames.

To combat this, AWMs are developed, leveraging transformers solely conditioned on sequences of actions, plus an initial state. This formulation inherently avoids the complexities of multistep state predictions, thus sidestepping the pitfalls associated with unstable gradients. Their AWM framework aligns policy gradients closely with underlying transformer architectures, offering a notable advantage in practical settings.

Theoretical Implications

The paper provides an analytical lens into the behavior of AWMs vs. conventional HWMs through theoretical bounds. It is found that AWMs, unlike HWMs, yield policy gradients that scale polynomially rather than exponentially with the horizon, showing a marked improvement in temporal credit assignment. AWMs extend the transformer’s favorable gradient properties into effective policy optimization, theoretically grounding their long-term efficacy.

Empirical Evaluation

Empirical validation is conducted across various challenging domains like the double-pendulum and tasks from the Myriad testbed. These experiments showcased AWMs outperforming both learned and simulator-based Markovian models, particularly when faced with chaotic or non-differentiable dynamics. Predictably, transformers as AWMs produced landscapes conducive to more efficient optimization, yielding superior policy performances against model-free and model-based baselines.

Future Perspectives

The findings suggest promising future developments in RL, particularly in scaling methods to accommodate even broader, more complex sequences and partially observable environments. By eschewing reliance on explicit state transitions and investing in the model's internal dynamics, there's potential to better handle the inherent uncertainties and exploratory challenges in real-world applications.

Conclusion

In synthesizing these insights, the paper convincingly argues the merit of AWMs over HWMs and conventional Markovian models in improving policy gradients. This work not only enriches the literature by bridging advanced sequence modeling techniques with RL but also provides a compelling framework for using transformers to address longstanding challenges in policy optimization. As such, it sets the stage for further exploration of deep sequence models in extended horizon RL tasks, a domain ripe for innovation and adaptation.