Remove prompts arg from WinrateCallback

[qgallouedec]

What does this PR do?

Fixes #1859

Remove "prompt" (not used). Instead, the prompts come from the eval dataset.

  def __init__(
       self,
-      prompts: List[str],
       judge: BaseRankJudge,
       trainer: Trainer,
       generation_config: Optional[GenerationConfig] = None,
       batch_size: int = 4,
  ):

I also add some tests and doc.

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