Add use_best_model feature for evaluating trials in hyperparameter searching
Author: jasper-lu
Pull request: yes

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Enables use of best epoch when evaluating trials in hyperparameter searching. Addresses feature request https://github.com/huggingface/transformers/issues/25247

I considered two approaches to adding this:
1. Add as a another flag in TrainingArguments.
2. (this one) Add by piping it through the `hyperparameter_search` method.

I ended up choosing (2) because hyperparameter searching seems like something "applied onto" training, rather than something intrinsic to training itself.

Once a reviewer confirms approach (2) looks good, I can write unit tests. Previously tested by setting `use_best_model=True` and inspecting values manually. Would also appreciate if anyone had thoughts on what the tests should look like.

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Comments:
@sgugger 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_25811). All of your documentation changes will be reflected on that endpoint.

> I don't think we want to touch that much in depth the hyperparameter search feature since we are not really maintaining it anymore. Unless @muellerzr or @pacman100 want to give it a second wind that is!

Gotcha. I'll just clean up this PR and put it into a good state in case you ever want to bring it in (and https://github.com/huggingface/transformers/issues/25247 can pull this in to support their use case).

> > I don't think we want to touch that much in depth the hyperparameter search feature since we are not really maintaining it anymore. Unless @muellerzr or @pacman100 want to give it a second wind that is!
> 
> Gotcha. I'll just clean up this PR and put it into a good state in case you ever want to bring it in (and #25247 can pull this in to support their use case).

Thank you @jasper-lu for you work and comprehension 🤗 ! Very appreciated ❤️ 

This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.