Horizon Generalization in Reinforcement Learning

We study goal-conditioned RL through the lens of generalization, but not in the traditional sense of random augmentations and domain randomization. Rather, we aim to learn goal-directed policies that generalize with respect to the horizon: after training to reach nearby goals (which are easy to learn), these policies should succeed in reaching distant goals (which are quite challenging to learn). In the same way that invariance is closely linked with generalization is other areas of machine learning (e.g., normalization layers make a network invariant to scale, and therefore generalize to inputs of varying scales), we show that this notion of horizon generalization is closely linked with invariance to planning: a policy navigating towards a goal will select the same actions as if it were navigating to a waypoint en route to that goal. Horizon generalization and invariance to planning are appealing because of their potential reach: they imply that a policy trained to reach nearby goals would succeed at reaching goals that are arbitrarily more distant.Our theoretical analysis proves that both horizon generalization and planning invariance are possible, under some assumptions. We present new experimental results, as well as recalling results from prior work, in support of our theoretical results. Taken together, our results open the door to studying how techniques for invariance and generalization developed in other areas of machine learning might be adapted to achieve this alluring property.