ModelHubMixin overwrite config if preexistant

[Wauplin]

Good catch from @not-lain. If a config file is saved by _save_pretrained, we don't want to overwrite it (done in #2102. But if a config file already exists before saving, we should overwrite it (as any other file). This PR fixes this + adds a test case.

(also removing a warning in a test)

[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.

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 82.69%. Comparing base (3252e27) to head (e248ada).

Additional details and impacted files

@@            Coverage Diff             @@
##             main    #2142      +/-   ##
==========================================
- Coverage   82.70%   82.69%   -0.01%     
==========================================
  Files         103      103              
  Lines        9628     9629       +1     
==========================================
  Hits         7963     7963              
- Misses       1665     1666       +1     

☔ View full report in Codecov by Sentry.
📢 Have feedback on the report? Share it here.