Incorrect typing of `debug` field in `TrainingArguments`
### System Info

https://github.com/huggingface/transformers/blob/v4.29.2/src/transformers/training_args.py#L862



### Who can help?

_No response_

### Information

- [ ] The official example scripts
- [X] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [X] My own task or dataset (give details below)

### Reproduction

If you use `TrainingArguments` and try to pass in a list of strings to `debug`, type checkers will complain.

### Expected behavior

The `debug` field should be typed as `Union[str, List[str]]` so type checkers don't complain.

For my particular use case, I am using a library that serializes dataclasses based on their type annotations, and it fails to serialize if `debug` is `[]` since the type hint says it is `str`. But the `__post_init__` turns `debug=""` into `debug=[]`, so there's no way I can get instances of this class to serialize without passing in non-empty debug options. I would gladly submit a PR myself, but I am not sure if it would have implications for the command line parsers.
The `debug` field does not accept a list of string, but a list of `DebugOption`, so the type annotation should be more like `Union[str, List[DebugOption]]`.

Happy to have a look at a PR if you want to fix it.