Train args defaulting None marked as Optional
# What does this PR do?

Fixes https://github.com/huggingface/transformers/issues/16701 by marking every `TrainingArguments` argument `Optional` if it currently defaults to `None`. Other applications which depend on static type hinting will no longer be confused by arguments defaulting to `None` although their declared type is `bool`.


## 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
_The documentation is not available anymore as the PR was closed or merged._