  Offline reinforcement learning (RL) can in principle synthesize more optimal
behavior from a dataset consisting only of suboptimal trials. One way that this
can happen is by "stitching" together the best parts of otherwise suboptimal
trajectories that overlap on similar states, to create new behaviors where each
individual state is in-distribution, but the overall returns are higher.
However, in many interesting and complex applications, such as autonomous
navigation and dialogue systems, the state is partially observed. Even worse,
the state representation is unknown or not easy to define. In such cases,
policies and value functions are often conditioned on observation histories
instead of states. In these cases, it is not clear if the same kind of
"stitching" is feasible at the level of observation histories, since two
different trajectories would always have different histories, and thus "similar
states" that might lead to effective stitching cannot be leveraged.
Theoretically, we show that standard offline RL algorithms conditioned on
observation histories suffer from poor sample complexity, in accordance with
the above intuition. We then identify sufficient conditions under which offline
RL can still be efficient -- intuitively, it needs to learn a compact
representation of history comprising only features relevant for action
selection. We introduce a bisimulation loss that captures the extent to which
this happens, and propose that offline RL can explicitly optimize this loss to
aid worst-case sample complexity. Empirically, we show that across a variety of
tasks either our proposed loss improves performance, or the value of this loss
is already minimized as a consequence of standard offline RL, indicating that
it correlates well with good performance.
