Add early stopping args to TrainingArguments
# What does this PR do?

While working in the collaborative training project, I added early stopping args to `TrainingArguments`.

Feel free to close this PR if you consider it is not pertinent.

## Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

@sgugger 
I reused the script:
- https://github.com/huggingface/transformers/blob/master/examples/pytorch/token-classification/run_ner.py

but allowing early stopping: [`EarlyStoppingCallback`](https://github.com/huggingface/transformers/blob/master/src/transformers/trainer_callback.py#L505)

Therefore, I had to add those args to `TrainingArguments` to allow end users of my script pass those parameters to EarlyStoppingCallback.

I thought that it might be useful for end users to have those args in TrainingArguments so that they can use early stopping in their trainer the same way I did for my script.<|||||>We don't have any `TrainingArguments` that are not used anywhere. Users are already complaining this class has too many, so if we had some they have to do something.
If they go with an update in the example scripts, then all the example scripts should be updated in the PR :-) <|||||>I could also add early stopping to some of the example scripts... 😅 I may do it this weekend though...<|||||>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.

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