Feat/onnx support

[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.

Fixes #553, fixes #2617.

Summary

• Small refactoring of the _process_input_batch and _get_batch_prediction() methods to make the behavior uniform across PLModules
• Added to_onnx method to the TorchForecastingModel class, taking care of creating the input_names and input_sample based on the model attributes
• Added a section in the User Guide to show how to export and load the model to generate forecasts

Other Information

The consistency of the forecasts was tested in a notebook for all the possible configurations, I did not implement unit-test as onnx should remain an optional dependency. Let me know what you think.

[codecov]

Codecov Report

Attention: Patch coverage is 39.39394% with 20 lines in your changes missing coverage. Please review.

Project coverage is 93.98%. Comparing base (71a1902) to head (22ea061).

Files with missing lines	Patch %	Lines
...arts/models/forecasting/torch_forecasting_model.py	5.55%	17 Missing ⚠️
darts/models/forecasting/pl_forecasting_module.py	75.00%	3 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2620      +/-   ##
==========================================
- Coverage   94.15%   93.98%   -0.18%     
==========================================
  Files         139      139              
  Lines       14992    15017      +25     
==========================================
- Hits        14116    14113       -3     
- Misses        876      904      +28     

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