New option called "best" for args.save_strategy.

[seanswyi]

What does this PR do?

Addresses #31626.

Adds a new option called "best" for TrainingArguments.save_strategy which saves the model checkpoint each time a new best performance is achieved.

Details

1. The previous _save_checkpoint method was in charge of not only saving the model checkpoint but also determining the best metric and best checkpoint. The logic for determining a new best metric was separated out into the _determine_best_metric method.
2. _determine_best_metric is called after every evaluation inside of _maybe_log_save_evaluate. The return value new_best_metric is used to determine whether or not a new best metric has been achieved, and if the save strategy is "best" then the TrainerControl's should_save flag is switched on.
   • Contrary to what I initially thought, best_metric does not seem to be tracked by default. Rather, it's only tracked when args.metric_for_best_model is provided. I believe that a best metric of some sort should always be tracked, and therefore if a value is not provided then the validation loss is used to determine a new best.
3. A new object called SaveStrategy was created in trainer_utils that adds a new attribute called BEST to the previous IntervalStrategy.

I'm not sure if I like the rather "hack-y" way that I implemented this by manually switching the TrainerControl's should_save flag rather than delegating it to the callback handler like the other flags are dealt with. The problem is that the flags are normally updated before calling _maybe_log_save_evaluate inside of the inner training loop, which means there's no way for us to determine whether or not a new best metric has been achieved with the current logic. I'm not sure if I'm making sense, but I'm open to any other suggestions.

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Who can review?

@muellerzr @SunMarc

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