Allow WinRateCallback to be used without reference model

[qgallouedec]

What does this PR do?

Fixes #1860

  class WinRateCallback(TrainerCallback):
      def __init__(self, ...):
          ...
-         if not hasattr(trainer, "ref_model"):
-             raise AttributeError("Trainer must have a `ref_model` attribute.")

      def on_train_begin(self, ...):
-         model = self.trainer.ref_model
+         model = getattr(self.trainer, "ref_model", kwargs["model"])

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