Efficient Planning in a Compact Latent Action Space (2208.10291v3)

Abstract: Planning-based reinforcement learning has shown strong performance in tasks in discrete and low-dimensional continuous action spaces. However, planning usually brings significant computational overhead for decision-making, and scaling such methods to high-dimensional action spaces remains challenging. To advance efficient planning for high-dimensional continuous control, we propose Trajectory Autoencoding Planner (TAP), which learns low-dimensional latent action codes with a state-conditional VQ-VAE. The decoder of the VQ-VAE thus serves as a novel dynamics model that takes latent actions and current state as input and reconstructs long-horizon trajectories. During inference time, given a starting state, TAP searches over discrete latent actions to find trajectories that have both high probability under the training distribution and high predicted cumulative reward. Empirical evaluation in the offline RL setting demonstrates low decision latency which is indifferent to the growing raw action dimensionality. For Adroit robotic hand manipulation tasks with high-dimensional continuous action space, TAP surpasses existing model-based methods by a large margin and also beats strong model-free actor-critic baselines.

Efficient Planning in a Compact Latent Action Space

The paper presents a new approach to planning-based reinforcement learning (RL) in high-dimensional continuous action spaces using a model named Trajectory Autoencoding Planner (TAP). The work is positioned in the context of reinforcement learning, where planning-based methods have traditionally excelled in discrete and low-dimensional settings but faced significant challenges when applied to high-dimensional action spaces due to computational inefficiencies.

Contributions

TAP builds upon the Vector Quantised Variational Autoencoder (VQ-VAE) framework to encode long-horizon trajectories into a compact latent space, acting as discrete latent actions. This approach provides several noteworthy propositions:

1. Latent Space Planning: The use of discrete latent action spaces derived from a state-conditioned encoder allows TAP to decouple the complexities of the high-dimensional raw action space from the planning process. Each latent action corresponds to a sequence of potential actions, thereby reducing the dimensionality and enhancing the efficiency of the planning.
2. Efficient Trajectory Sampling: The model leverages an autoregressive transformer to model the prior distribution over latent code sequences—enabling planning through beam search in this compact action space. This facilitation of multiple-step prediction with a single latent action allows significant speed-ups in decision-making processes.
3. Handling High-Dimensional Control Tasks: By reconstructing entire trajectories, TAP effectively reduces compounded errors commonly encountered in step-by-step rollouts within traditional dynamics models. Empirical evaluations demonstrate the robustness of TAP, showing superior performance against state-of-the-art model-based and model-free methods across several D4RL benchmarks, including Adroit robotic hand manipulation tasks.

Experimental Results

The paper provides empirical validation of TAP through extensive experiments in both low-dimensional locomotion tasks and high-dimensional robotics tasks, addressing a wide spectrum of action and state spaces. Key findings include:

• Strong Numerical Performance: TAP consistently surpasses model-based and model-free baselines, especially in high-dimensional environments like Adroit, showcasing its potential in managing complex control tasks.
• Computation Efficiency: The decision latency associated with TAP is shown to remain stable regardless of growing action-space dimensionality, contrasting sharply with alternative models like Trajectory Transformer (TT), whose latency scales unfavorably with complexity.

Theoretical and Practical Implications

The adoption of latent action spaces paves the way for more scalable and flexible RL systems capable of handling intricate dynamics and large state-action spaces. From a theoretical standpoint, this strategy highlights the potential of integrating advanced sequence modeling techniques (Transformers) into latent-space dynamics modeling for RL, promising future exploration.

Practically, the reduced computational overhead and better scalability make TAP a strong contender for real-time applications in robotics and autonomous systems. This improvement in latency and computational demand meets the operational constraints of current hardware, enabling broader deployment in practical scenarios.

Future Developments

Future research directions may explore extended applicability of TAP to stochastic environments where the distinction between model uncertainty and policy stochasticity becomes paramount. Moreover, further abstraction or adaptability of the latent space could enhance generalization across even more diverse tasks without necessitating frequent retraining.

In conclusion, this paper introduces significant advancements in the field of planning-based reinforcement learning through compact latent action spaces, demonstrating notable performance and efficiency gains in high-dimensional control tasks. The adoption of TAP has clear implications for enhancing real-time decision-making in complex environments, making it a valuable contribution to the ongoing development of RL methodologies.