Reward Ensemble for RewardActor (Pre-Weaver)

[hgKang02]

Ensembles the reward (scores) given by different reward models (verifiers).

Changes

• hf_rm.py: HF reward model wrapper
• ensemble.py: ensemble reward aggregation
• rm_models.py: utility helpers for model loading

I've made it work for both non-reward model training and use-reward model training so that the user can decide whether to use the reward model or not.

The only thing you need to change is set use_model: true and add the models that you want to use as reward models.

Later I will add weaver based on this codebase.

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