Fix commonpath in read-only filesystem

[stevelaskaridis]

Description

When downloading a model, commonpath is defined as the LCA (Lowest Common Ancestor) of huggingface's cachedir and target output directory. When this ends-up being a directory that is not read-only (e.g. /), an OSError (errno: 30) is thrown.

This PR catches that error and tries to check symlink creation in the destination directory.

To reproduce

model_path = snapshot_download(repo_id=model,
                               local_dir='/tmp',
                               cache_dir=None, # TRANSFORMERS_CACHE default (i.e. ~/.cache/huggingface/hub/)
                               token=os.environ.get('HF_TOKEN', None))

Original behaviour

Tested on MacOS 14.1.2 and python 3.11.4, the output was:

OSError: [Errno 30] Read-only file system: '/tmpzeu59eox'

New behaviour

The model is persisted as intended.

[stevelaskaridis]

I am not even sure why symlinks are originally tested on commonpath, but tried to stay as close as possible to the initial functionality.

[Wauplin]

Makes sense yes, thanks for reporting the issue and fixing it in a PR!
For the record, we might not need this fix in a near future once we revampt to_local_dir logic in #1738.

[HuggingFaceDocBuilderDev]

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