Planning to Practice: Efficient Online Fine-Tuning by Composing Goals in Latent Space

General-purpose robots in real-world settings require diverse repertoires of behaviors to complete challenging tasks in unstructured environments. To address this problem, goal-conditioned reinforcement learning aims to train policies that can reach configurable goals for a wide range of tasks on command. However, such goal-conditioned policies are notoriously difficult and time-consuming to train from scratch. In this paper, we propose Planning to Practice (PTP), a method that makes it practical to train goal-conditioned policies for long-horizon tasks that require multiple distinct types of interactions to solve. Our approach is based on two key ideas. First, we decompose the goal-reaching problem hierarchically, with a high-level planner that sets intermediate subgoals using conditional subgoal generators in the latent space for a low-level model-free policy. Second, we propose a hybrid offline reinforcement learning approach with online fine-tuning, which uses previously collected data to pre-train both the conditional subgoal generator and the policy, and then fine-tune the policy via online exploration. This fine-tuning process is itself facilitated by the planned subgoals, which break down the original target task into short-horizon goal-reaching tasks that are significantly easier to learn. We conduct experiments in both the simulation and real world, in which the policy is pre-trained on demonstrations of short primitive behaviors and fine-tuned for temporally extended tasks that are unseen in the offline data. Our experimental results show that PTP can generate feasible sequences of subgoals that enable the policy to efficiently solve the target tasks.