[AutoModel] Fix bug with subfolders and local model paths when loading custom code

[DN6]

What does this PR do?

When loading custom code from a local path with AutoModel, the subfolder argument is not properly handled in get_cached_module_file. This results in this check for a valid file path failing and a local path being passed to hf_hub_download, which throws an error.

diffusers/src/diffusers/utils/dynamic_modules_utils.py

Line 304 in 3918824

if os.path.isfile(module_file_or_url):

e.g. This currently fails

model = AutoModel.from_pretrained("<my_local_path>", subfolder="transformer", trust_remote_code=True)

but this works because the full local path is provided.

model = AutoModel.from_pretrained("<my_local_path>/transformer", trust_remote_code=True)

This PR adds a check to handle subfolder properly when fetching local files.

Fixes # (issue)

Before submitting

• This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
• Did you read the contributor guideline?
• Did you read our philosophy doc (important for complex PRs)?
• Was this discussed/approved via a GitHub issue or the forum? Please add a link to it if that's the case.
• Did you make sure to update the documentation with your changes? Here are the
  documentation guidelines, and
  here are tips on formatting docstrings.
• Did you write any new necessary tests?

Who can review?

Anyone in the community is free to review the PR once the tests have passed. Feel free to tag
members/contributors who may be interested in your PR.

[HuggingFaceDocBuilderDev]

The docs for this PR live here. All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

[yiyixuxu]

thanks!