Fix HfArgumentParser parsing for Union[dict, str] fields (e.g., --lr_scheduler_kwargs)
Author: gmmkmtgk
Pull request: yes

…scheduler_kwargs)

# What does this PR do?

This PR fixes the bug where `Union[dict, str]` fields, such as `--lr_scheduler_kwargs` in `TrainingArguments`, could not be parsed correctly from the command line.

**Root Cause:**
The issue was introduced when the type of `lr_scheduler_kwargs` changed from `Optional[Union[dict[str, Any], str]]` to `Union[dict[str, Any], str]`. `HfArgumentParser` previously filtered out `str` from Union types when `None` was not present, which caused `argparse` to attempt parsing the argument as a `dict` directly, leading to errors when passing JSON strings via CLI.

**Fix:**

* Modified `src/transformers/hf_argparser.py` to detect `Union[dict, str]` types and keep the type as `str` for `argparse`.
* Conversion from JSON string → `dict` now happens later in `TrainingArguments.post_init_`.
* Maintains backward compatibility with `Optional[Union[dict, str]]` fields.

**Tests added:**

* `test_17_union_dict_str_parsing()`: verifies basic Union[dict, str] CLI parsing.
* `test_18_lr_scheduler_kwargs_parsing()`: regression test specifically for the `lr_scheduler_kwargs` issue.

Fixes the behavior reported in [#41296](https://github.com/huggingface/transformers/issues/41296).


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Fixes # (issue)


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Comments:
cc @sunmarc I think!

@SunMarc any way this contribution can be counted in hacktoberfest?