fix(pip): Pass --abi flag when downloading wheel for freethreaded python

[meteorcloudy]

When pip download is used for fetching pip dependency, we need to pass --abi flag to fetch the correct wheel file for freethreaded python.

Essentially making https://github.com/google-ml-infra/rules_ml_toolchain/blob/1c2352bde159ee14cc5050a871c437622fe25e1f/py/python_init_pip.bzl#L69-L71 unnecessary.

[gemini-code-assist]

Code Review

This pull request introduces logic to automatically append the correct --abi flag for freethreaded Python platforms when download_only is enabled. This is achieved by passing the download_only and python_version attributes through the hub builder to the requirement parser. A review comment suggests optimizing the implementation by avoiding redundant list copies and refining the check for existing --abi flags.

[gemini-code-assist]

The current implementation creates a new list copy for every platform, even when no modifications are needed. This can be optimized by only copying the list if the conditions for adding the --abi flag are met. Additionally, the check for existing --abi flags can be made more precise to avoid matching other flags that might start with the same prefix (though unlikely in this context).

Suggested change

	plat_args = list(pip_args)
	if download_only and plat.endswith("freethreaded") and python_version:
	if not any([a.startswith("--abi") for a in plat_args]):
	major, _, tail = python_version.partition(".")
	minor, _, _ = tail.partition(".")
	if major and minor:
	plat_args.append("--abi=cp{}{}t".format(major, minor))
	options[plat] = plat_args
	plat_args = pip_args
	if download_only and plat.endswith("freethreaded") and python_version:
	if not any([a == "--abi" or a.startswith("--abi=") for a in plat_args]):
	major, _, tail = python_version.partition(".")
	minor, _, _ = tail.partition(".")
	if major and minor:
	plat_args = list(pip_args)
	plat_args.append("--abi=cp{}{}t".format(major, minor))
	options[plat] = plat_args

[aignas]

With 2.0.0 and above this might not be needed, because by default we are using the downloader to fetch the wheels and the freethreaded wheels should be supported by default.