Weighted-Reward Preference Optimization for Implicit Model Fusion
Authors: name: Ziyi Yang, name: Fanqi Wan, name: Longguang Zhong, name: Tianyuan Shi, name: Xiaojun Quan

While fusing heterogeneous open-source LLMs with varying architectures and
sizes can potentially integrate the strengths of different models, existing
fusion methods face significant challenges, such as vocabulary alignment and
merging distribution matrices. These procedures are not only complex but also
prone to introducing noise and errors. In this paper, we propose an implicit
fusion method, Weighted-Reward Preference Optimization (WRPO), which leverages
preference optimization between the source LLMs and the target LLM to transfer
their capabilities effectively. WRPO eliminates the need for vocabulary
alignment and matrix fusion and can be efficiently scaled to accommodate
various LLMs. To address distributional deviations between the source and
target LLMs, WRPO introduces a progressive adaptation strategy that gradually
shifts reliance on preferred examples from the target LLM to the source LLMs.
Extensive experiments on the MT-Bench, AlpacaEval-2, and Arena-Hard benchmarks
demonstrate that WRPO consistently outperforms existing knowledge fusion
methods and various fine-tuning baselines. When applied to LLaMA3-8B-Instruct
as the target model, WRPO achieves a length-controlled win rate of 55.9%
against GPT-4-Preview-1106 on AlpacaEval-2 and a win rate of 46.2% against
GPT-4-0314 on Arena-Hard. Our code is available at
https://github.com/SLIT-AI/WRPO.