fix type annotations for arguments in training_args
# What does this PR do?

Fixes #24538 which is basically fixing type annotations for `fsdp`, `fsdp_config`, and `sharded_ddp` in training_args.py

## Who can review?
maybe @sgugger 
Fixing CI errors! <|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger I don't particularly understand why this error occurs in examples_flax - ``` argparse.ArgumentError: argument --sharded_ddp: invalid typing.Union[str, bool, typing.List[transformers.trainer_utils.ShardedDDPOption], NoneType] value: '' ```<|||||>@sgugger `bool` breaks `--sharded_ddp`, I think we can still maintain Boolean arguments with string itself and 
https://github.com/huggingface/transformers/blob/20d6b84613984f2497587a62774704882ccbeee6/src/transformers/hf_argparser.py#L168-L173
with this `--sharded_ddp` and `--fsdp` defaults to string <|||||>The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_24550). All of your documentation changes will be reflected on that endpoint.