Why so pessimistic? Estimating uncertainties for offline rl through ensembles, and why their independence matters

In offline/batch reinforcement learning (RL), the predominant class of approaches with most success have been ``support constraint" methods, where trained policies are encouraged to remain within the support of the provided offline dataset. However, support constraints correspond to an overly pessimistic assumption that actions outside the provided data may lead to worst-case outcomes. In this work, we aim to relax this assumption by obtaining uncertainty estimates for predicted action values, and acting conservatively with respect to a lower-confidence bound (LCB) on these estimates. Motivated by the success of ensembles for uncertainty estimation in supervised learning, we propose MSG, an offline RL method that employs an ensemble of independently updated Q-functions. First, theoretically, by referring to the literature on infinite-width neural networks, we demonstrate the crucial dependence of the quality of derived uncertainties on the manner in which ensembling is performed, a phenomenon that arises due to the dynamic programming nature of RL and overlooked by existing offline RL methods. Our theoretical predictions are corroborated by pedagogical examples on toy MDPs, as well as empirical comparisons in benchmark continuous control domains. In the significantly more challenging antmaze domains of the D4RL benchmark, MSG with deep ensembles by a wide margin surpasses highly well-tuned state-of-the-art methods. Consequently, we investigate whether efficient approximations can be similarly effective. We demonstrate that while some very efficient variants also outperform current state-of-the-art, they do not match the performance and robustness of MSG with deep ensembles. We hope that the significant impact of our less pessimistic approach engenders increased focus into uncertainty estimation techniques directed at RL, and engenders new efforts from the community of deep network uncertainty estimation researchers.