Fix TrainingArgs argument serialization
# What does this PR do?
Filter out arguments in TrainingArguments.to_dict() such as `field(init=False)` so arguments such as `_n_gpu` won't be serialized.

Fixes #19236 

## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
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      Pull Request section?
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      to it if that's the case.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [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?
@sgugger 
_The documentation is not available anymore as the PR was closed or merged._