Feat/test optional dep (onnx, ray, optuna)

[madtoinou]

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog.

Fix #2651.

Summary

• Add a optional.txt file and update the github action workflow accordingly (cache update)
• Added tests for the ray, onnx, and optuna optional dependencies
• The tests of the optional dependencies are included in the coverage check of the "all" darts flavor.
• Fix a bug in load_weights_from_checkpoint, where the use_X_covariates were not properly updated

Other Information

• The tests for the optional dependencies were heavily inspired from the example contained in the User Guide, should make it easier to maintain the documentation
• The tests for polars and other optional libraries providing dataframe-like data structure introduced with Narwhals implementation of from_dataframe and performance benchmark #2661 could leverage the same logic.

[codecov]

Codecov Report

Attention: Patch coverage is 81.63265% with 9 lines in your changes missing coverage. Please review.

Project coverage is 94.22%. Comparing base (19b17d2) to head (2a7c268).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...arts/models/forecasting/torch_forecasting_model.py	68.96%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2702      +/-   ##
==========================================
+ Coverage   94.18%   94.22%   +0.03%     
==========================================
  Files         141      142       +1     
  Lines       15598    15647      +49     
==========================================
+ Hits        14691    14743      +52     
+ Misses        907      904       -3     

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[dennisbader]

Very nice PR, thanks a lot @madtoinou 🚀

Just had some minor suggestions