Scalable Ensembling For Mitigating Reward Overoptimisation (2406.01013v2)

Abstract: Reinforcement Learning from Human Feedback (RLHF) has enabled significant advancements within LLMing for powerful, instruction-following models. However, the alignment of these models remains a pressing challenge as the policy tends to overfit the learned proxy" reward model past an inflection point of utility as measured by agold" reward model that is more performant -- a phenomenon known as overoptimisation. Prior work has mitigated this issue by computing a pessimistic statistic over an ensemble of reward models, which is common in Offline Reinforcement Learning but incredibly costly for LLMs with high memory requirements, making such approaches infeasible for sufficiently large models. To this end, we propose using a shared encoder but separate linear heads. We find this leads to similar performance as the full ensemble while allowing tremendous savings in memory and time required for training for models of similar size.