Fixing save_pretrained to comply with huggingface_hub v0.21

[Natooz]

Since huggingface_hub v0.21 ModelHubMixin.save_pretrained will automatically save self.config after calling self._save_pretrained, which is unnecessary in our case as self._save_pretrained already saves the tokenizer's config.

This PR overrides ModelHubMixin.save_pretrained to get rid of the config logic.

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📚 Documentation preview 📚: https://miditok--150.org.readthedocs.build/en/150/

[codecov]

Codecov Report

Attention: Patch coverage is 63.63636% with 4 lines in your changes are missing coverage. Please review.

Project coverage is 91.02%. Comparing base (18a6b3a) to head (3ca4190).
Report is 1 commits behind head on main.

Files	Patch %	Lines
miditok/midi_tokenizer.py	60.00%	4 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #150      +/-   ##
==========================================
- Coverage   91.15%   91.02%   -0.13%     
==========================================
  Files          33       33              
  Lines        4984     4994      +10     
==========================================
+ Hits         4543     4546       +3     
- Misses        441      448       +7     

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