Sequence Model Imitation Learning with Unobserved Contexts

We consider imitation learning problems where the learner's ability to mimic the expert increases throughout the course of an episode as more information is revealed. One example of this is when the expert has access to privileged information: while the learner might not be able to accurately reproduce expert behavior early on in an episode, by considering the entire history of states and actions, they might be able to eventually identify the hidden context and act as the expert would. We prove that on-policy imitation learning algorithms (with or without access to a queryable expert) are better equipped to handle these sorts of asymptotically realizable problems than off-policy methods. This is because onpolicy algorithms provably learn to recover from their initially suboptimal actions, while off-policy methods treat their suboptimal past actions as though they came from the expert. This often manifests as a latching behavior: a naive repetition of past actions. We conduct experiments in a toy bandit domain that show that there exist sharp phase transitions of whether off-policy approaches are able to match expert performance asymptotically, in contrast to the uniformly good performance of on-policy approaches. We demonstrate that on several continuous control tasks, on-policy approaches are able to use history to identify the context while off-policy approaches actually perform worse when given access to history. Recent theoretical work has established "no-go" results for successfully imitating an expert that has access to more information [Zhang et al., 2020 , Kumor et al., 2021] . The core of their arguments is that without seeing some feature that influences expert behavior but is not echoed elsewhere in the state, the learner might not be able to properly ground the expert actions in the observed state. In causal inference terms, this hidden information acts as an unobserved confounder which prevents identification of the desired causal estimand (the expert action). Despite these results, 36th Conference on Neural Information Processing Systems (NeurIPS 2022).