argparser: Allow optional bool flags without values

[antznette1]

Summary

Improve HfArgumentParser boolean UX so Optional[bool] arguments can be passed as flags without an explicit value.

What changed

• Updated HfArgumentParser to treat typing.Optional[bool] (Union[bool, None]) the same as bool | None:
  • keeps default None
  • allows --flag (no value) to parse as True via nargs="?" + const=True
• Updated unit tests in tests/utils/test_hf_argparser.py to reflect the new argparse configuration and verify --opt works without an explicit value.

[Rocketknight1]

Is there an issue or reproducer to show the bug that this is fixing..?

[antznette1]

Is there an issue or reproducer to show the bug that this is fixing..?

This is fixing a CLI UX bug in HfArgumentParser for Optional[bool].
Before this PR, an Optional[bool] flag behaves like a value-taking option, so passing the flag without a value errors (argparse expects an argument), even though most CLIs treat --flag as “enable”.

Reproducer:

from dataclasses import dataclass
from typing import Optional
from transformers.hf_argparser import HfArgumentParser
@DataClass
class Args:
opt: Optional[bool] = None
parser = HfArgumentParser(Args)
parser.parse_args_into_dataclasses(["--opt"]) # previously errors, now sets opt=True

This PR makes Optional[bool] behave like a standard boolean flag: --opt sets True, while still allowing explicit values (--opt false) and preserving the default None.