Trainer: fixed evaluate raising KeyError for ReduceLROnPlateau

[claudius-kienle]

If the metric_for_best_model does not start with eval_, the trainer always appends the eval_ string before querying the metric value. Except in one code snippet, that only gets triggered if one uses the ReduceLROnPlateau scheduler.
I have added the check for eval like in the rest of the trainer.

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[sgugger]

Thanks!

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