Docs update

[Natooz]

split_files_for_training , split_score_per_note_density, get_average_num_tokens_per_note and split_dataset_to_subsequences methods moved from the "PyTorch_data" module to the "utils" module as it doesn't have to rely on PyTorch and should be able to be used with any DL framework.

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📚 Documentation preview 📚: https://miditok--175.org.readthedocs.build/en/175/

[codecov]

Codecov Report

Attention: Patch coverage is 86.20690% with 8 lines in your changes missing coverage. Please review.

Project coverage is 90.53%. Comparing base (5b3c3f6) to head (e3da69e).
Report is 2 commits behind head on main.

❗ Current head e3da69e differs from pull request most recent head 21ab591

Please upload reports for the commit 21ab591 to get more accurate results.

Files	Patch %	Lines
miditok/classes.py	75.86%	7 Missing ⚠️
miditok/pytorch_data/collators.py	50.00%	1 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##             main     #175      +/-   ##
==========================================
+ Coverage   90.46%   90.53%   +0.07%     
==========================================
  Files          35       35              
  Lines        5525     5527       +2     
==========================================
+ Hits         4998     5004       +6     
+ Misses        527      523       -4     

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