Value-Incentivized Preference Optimization: A Unified Approach to Online and Offline RLHF (2405.19320v3)

Abstract: Reinforcement learning from human feedback (RLHF) has demonstrated great promise in aligning LLMs with human preference. Depending on the availability of preference data, both online and offline RLHF are active areas of investigation. A key bottleneck is understanding how to incorporate uncertainty estimation in the reward function learned from the preference data for RLHF, regardless of how the preference data is collected. While the principles of optimism or pessimism under uncertainty are well-established in standard reinforcement learning (RL), a practically-implementable and theoretically-grounded form amenable to LLMs is not yet available, as standard techniques for constructing confidence intervals become intractable under arbitrary policy parameterizations. In this paper, we introduce a unified approach to online and offline RLHF -- value-incentivized preference optimization (VPO) -- which regularizes the maximum-likelihood estimate of the reward function with the corresponding value function, modulated by a $\textit{sign}$ to indicate whether the optimism or pessimism is chosen. VPO also directly optimizes the policy with implicit reward modeling, and therefore shares a simpler RLHF pipeline similar to direct preference optimization. Theoretical guarantees of VPO are provided for both online and offline settings, matching the rates of their standard RL counterparts. Moreover, experiments on text summarization and dialog verify the practicality and effectiveness of VPO.

Value-Incentivized Preference Optimization for RLHF

Introduction

The paper "Reinforcement learning from human feedback (RLHF)" presents an approach to refine LLMs by aligning their outputs with human preferences. The novelty lies in a technique termed Value-Incentivized Preference Optimization (VPO), which regularizes the reward function derived from preference data with accompanying value functions. This method unifies both online and offline RLHF and offers theoretical and practical advancements.

Key Contributions

1. Integration of Optimism/Pessimism: The central innovation, VPO, directly integrates the optimism/pessimism principles for handling uncertainty. Instead of the traditional need for explicit uncertainty estimation, VPO modulates the maximum likelihood estimate (MLE) of the reward function by employing a value-based regularization term. This allows for implicit estimation of uncertainty in a computationally feasible manner, a first for LLMs.
2. Combining Online and Offline RLHF: VPO effectively bridges online and offline RLHF. For the online setting, its strategy involves iteratively collecting new preference data and refining the reward and policy models iteratively. In the offline setting, it employs a single optimization pass over a pre-collected dataset, thus avoiding overfitting.
3. Theoretical Guarantees: The paper offers robust theoretical guarantees for VPO, showcasing regret bounds in online settings comparable to those seen in traditional contextual bandit problems. Similarly, for offline scenarios, VPO's performance converges optimally, aligning with state-of-the-art benchmarks under linear function approximation.

Implications and Numerical Results

The practical implications of VPO are significant:

• Efficiency in Training Pipelines: By circumventing the need for confidence interval constructions in arbitrarily parameterized policies, VPO simplifies the RLHF pipeline. This makes it an attractive option for real-world applications where resource constraints are critical.
• Robust Performance: Empirical results reinforce VPO's effectiveness. On tasks like text summarization and dialog generation, VPO consistently outperformed baselines, both in terms of reward calibration and policy refinement.

Detailed Analysis

Online RLHF

The iterative procedure for online VPO involves three primary steps per iteration:

1. Sampling and Data Generation: New preference data is sampled using the current policy.
2. Reward Update: The reward function is updated by minimizing a regularized log-likelihood function, which includes an incentivizing term for the value (optimistic for online settings).
3. Policy Update: The policy is fine-tuned to maximize the reward function.

The paper's theoretical analysis demonstrates that online VPO achieves cumulative regret bounds of $\widetilde{\mathcal{O}}(\sqrt{T})$, aligning it closely with standard approaches in contextual bandits.

Offline RLHF

For offline settings, VPO operates in a single-shot manner using a pre-collected dataset:

1. Reward Model Learning: The reward model is refined using a dataset by integrating a term that discourages over-optimization.
2. Optimal Policy Update: The learned reward function is then employed to update the policy optimally.

Theoretical guarantees show VPO achieving sub-optimality gap rates of $\widetilde{\mathcal{O}}(1/\sqrt{N})$, where $N$ is the dataset size, underlining its efficacy in offline RLHF tasks.

Experimental Validation

Results from synthetic multi-armed bandit setups and real-world LLM tasks like ARC-Challenge and TL;DR showcase VPO's superior performance. Notably:

• Online Settings:

VPO maintained superior performance over multiple iterations, with sustained improvements over SFT.

• Offline Settings:

VPO avoided over-optimization pitfalls inherent in other methods, maintaining high performance across different model scales (e.g., Llama2, Flan-T5).

Future Directions

This work opens several avenues for further research:

1. Adaptive Regularization: Investigating adaptive strategies for choosing the regularization coefficient $\alpha$ could lead to even more efficient training procedures.
2. Extension to Broader RL Frameworks: The principles established here for VPO could be extended to other RL contexts, potentially redefining strategies for uncertainty-based optimization without explicit estimations.

Conclusion

Value-Incentivized Preference Optimization (VPO) addresses a critical challenge in RLHF by integrating uncertainty management directly into the reward function optimization process. The paper provides both theoretical assurances and practical validation, making VPO a promising addition to RLHF methodologies for LLMs. This contributes to advancing efficient and robust alignment of LLMs with human preferences.