Fix/hist fc predict kwargs

[dennisbader]

Fixes #2097.

Summary

• fixes a bug where some fit_kwargs and predict_kwargs were ignored in historical forecasts when the model's fit/predict methods had *args and **kwargs in the signature
• moved predict logic from mixed covariates models to MixedCovariatesTorchForecastingModels -> N/DLinear now use the proper logic
• fixed some text in docs

[codecov-commenter]

Codecov Report

Attention: 2 lines in your changes are missing coverage. Please review.

Comparison is base (7cfdf62) 93.91% compared to head (8c429b6) 93.89%.
Report is 1 commits behind head on master.

Files	Patch %	Lines
darts/models/forecasting/forecasting_model.py	96.55%	1 Missing ⚠️
...arts/models/forecasting/torch_forecasting_model.py	83.33%	1 Missing ⚠️

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2103      +/-   ##
==========================================
- Coverage   93.91%   93.89%   -0.02%     
==========================================
  Files         135      135              
  Lines       13299    13289      -10     
==========================================
- Hits        12490    12478      -12     
- Misses        809      811       +2     

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[madtoinou]

LGTM, thanks @dennisbader for the fix!