Add str to TrainingArguments report_to type hint
Author: ringohoffman
Pull request: yes

# What does this PR do?
Add `str` to `TrainingArguments`'s `report_to` type hint.

From the docstring:

```
        report_to (`str` or `List[str]`, *optional*, defaults to `"all"`):
```

## Before submitting
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      Pull Request section?
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      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

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Comments:
Hi @ringohoffman, thanks for opening this PR! 

The TF tests - `tests_tf` and `tests_pipelines_tf` - aren't related to this PR. We're working on a fix and will let you know when that's pushed to main and resolved. 

The other failing tests seem to be coming from this PR - they'll need to be resolved. I suspect you'll have to do something [like this](https://github.com/huggingface/transformers/blob/a907a903d644f835a5563b1133b1fc961e33c3d2/src/transformers/training_args.py#L1375) to account for the possible None type 

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_30078). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.