Only lock when actually downloading a model and per model_path

[simo-prior]

Issue

https://linear.app/priorlabs/issue/ENG-509/filenotfounderror-level-error-errno-2-no-such-file-or-directory-tabpfn

Motivation and Context

Previously we were applying a global lock on every load_model_criterion_config() call to prevent race conditions while downloading models. This call would not necessarily result in a download and different model_paths do not require a shared lock.

[chatgpt-codex-connector]

Codex usage limits have been reached for code reviews. Please check with the admins of this repo to increase the limits by adding credits.
Credits must be used to enable repository wide code reviews.

[CLAassistant]

All committers have signed the CLA.

[gemini-code-assist]

Code Review

This pull request refactors the model download locking mechanism from a single global lock to a more granular, per-file lock, which is a great improvement for concurrent downloads of different models. However, a security audit identified a medium-severity path traversal vulnerability in the download locking mechanism, which could be exploited for Denial of Service attacks. Additionally, this change will likely break the existing concurrency test test__load_model_criterion_config__parallel_downloads_do_not_crash because the @_with_download_lock decorator was moved from load_model_criterion_config to download_model, and the test currently mocks download_model directly, bypassing the new locking mechanism. The test needs to be updated to correctly verify the new locking mechanism. A suggestion has also been provided to improve the robustness of the new decorator.

[noahho]

I didn't go very deep but this looks good to me and very well engineeered