fix snapshot download behavior in offline mode when downloading to a local dir

[hanouticelina]

Should fix the incorrect behavior mentioned in this slack message (internal).

This PR addresses an issue where snapshot_download behaves incorrectly when:

• a user specifies a local_dir parameter in offline mode (either viia HF_HUB_OFFLINE=1 or connection error).
• the files are available in the cache but not in the local_dir.

Currently, in this scenario, we incorrectly returns the path to the cache directory instead of raising an appropriate error. This PR fixes that.

[HuggingFaceDocBuilderDev]

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[Wauplin]

From my understanding, I feel a correct fix would be to update L203 from

if commit_hash is not None:

to

if commit_hash is not None and local_dir is None:

i.e. we do the snapshot folder check only if local_dir is not provided by the user. When local_dir is set, the logic to handle things is a bit below (here) and returns with a warning if folder is not empty.

[Wauplin]

Thanks!