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Token-level Direct Preference Optimization (2404.11999v5)

Published 18 Apr 2024 in cs.CL and cs.AI
Token-level Direct Preference Optimization

Abstract: Fine-tuning pre-trained LLMs is essential to align them with human values and intentions. This process often utilizes methods like pairwise comparisons and KL divergence against a reference LLM, focusing on the evaluation of full answers generated by the models. However, the generation of these responses occurs in a token level, following a sequential, auto-regressive fashion. In this paper, we introduce Token-level Direct Preference Optimization (TDPO), a novel approach to align LLMs with human preferences by optimizing policy at the token level. Unlike previous methods, which face challenges in divergence efficiency, TDPO incorporates forward KL divergence constraints for each token, improving alignment and diversity. Utilizing the Bradley-Terry model for a token-based reward system, TDPO enhances the regulation of KL divergence, while preserving simplicity without the need for explicit reward modeling. Experimental results across various text tasks demonstrate TDPO's superior performance in balancing alignment with generation diversity. Notably, fine-tuning with TDPO strikes a better balance than DPO in the controlled sentiment generation and single-turn dialogue datasets, and significantly improves the quality of generated responses compared to both DPO and PPO-based RLHF methods. Our code is open-sourced at https://github.com/Vance0124/Token-level-Direct-Preference-Optimization.

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Token-level Direct Preference Optimization: Enhancing LLM Alignment with Human Preferences

Introduction

LLMs have become central to contemporary AI research due to their ability to generalize across various textual tasks. Traditional methods such as Direct Preference Optimization (DPO) and Reinforcement Learning from Human Feedback (RLHF) have made significant strides in aligning these models with human-like responses. However, these approaches often assess divergence at a sentence level and may not effectively control the token-level generation diversity. This paper introduces Token-level Direct Preference Optimization (TDPO), a novel approach focusing on token-level optimization to improve alignment and manage generative diversity more efficiently.

Advances in Direct Preference Optimization

TDPO offers a conceptual leap from traditional DPO by shifting the focus from sentence-level evaluation to token-level optimization. This method integrates forward KL divergence constraints at the token level, allowing for finer control over the model's output, aligning more closely with human preferences, and preserving generative diversity. This paper leverages the Bradley-Terry model for token-based preference assessment, enhancing traditional methods without the necessity for explicit reward model training or policy sampling during training.

Experimental Framework and Results

The authors conducted extensive experiments to validate TDPO's effectiveness across different datasets, including IMDb for sentiment generation and Anthropic HH for single-turn dialogues. The results were impressive, showing that TDPO outperforms both DPO and PPO-based RLHF methods in generating quality responses. Specifically, TDPO achieved better balance and control over KL divergence and showcased superior divergence efficiency.

Key insights include:

  • Alignment and Diversity: TDPO significantly improves the balance between model alignment with human preferences and generative diversity compared to existing methods.
  • KL Divergence Control: By optimizing at the token level, TDPO provides a more nuanced control of KL divergence, leading to more stable and consistent model performance across different textual tasks.

Implications and Future Directions

The introduction of TDPO marks a pivotal development in the training of LLMs. Looking forward, this method opens new avenues for research into fine-tuning LLMs at a more granular level. Future work may explore the potential of token-level optimizations in other aspects of LLMing, such as reducing toxicity or bias in generated texts. Additionally, further research could extend this approach to other forms of media, like generative models for audio or video, where fine-grained control over generative processes is crucial.

Conclusion

Token-level Direct Preference Optimization represents a significant refinement over sentence-level optimization techniques used in LLMs. By effectively addressing the challenges of divergence efficiency and maintaining a balance between alignment and diversity, TDPO sets a new standard for the development of human-aligned AI systems. The method's ability to fine-tune generative attributes at a token level will likely influence future LLM research and applications, making it a cornerstone in the ongoing evolution of machine learning models.

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Authors (6)
  1. Yongcheng Zeng (5 papers)
  2. Guoqing Liu (42 papers)
  3. Weiyu Ma (5 papers)
  4. Ning Yang (49 papers)
  5. Haifeng Zhang (58 papers)
  6. Jun Wang (990 papers)
Citations (20)
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