FIX: enable load_best_model_at_end within SaveStrategy.BEST and initialize metric_for_best_model as loss when SaveStrategy.BEST

[thechaos16]

What does this PR do?

• enable load_best_model_at_end when even if save_strategy is best
  • beforehand, it doesn't show any error, but it doesn't load the best, but the last (because self.state.best_global_step wasn't updated)
• enable metric_for_best_model when save_strategy is best
  • metric_for_best_model is initialized as loss when load_best_model_at_end is enabled, but not if save_strategy is set as best. I think these two behaviors should be the same.

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@zach-huggingface, @SunMarc and @qgallouedec