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 52.00000% with 12 lines in your changes missing coverage. Please review.

Project coverage is 94.12%. Comparing base (3aa5c97) to head (17711b7).
Report is 1 commits behind head on master.

Files with missing lines	Patch %	Lines
...arts/models/forecasting/torch_forecasting_model.py	7.69%	12 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2620      +/-   ##
==========================================
- Coverage   94.27%   94.12%   -0.15%     
==========================================
  Files         141      141              
  Lines       15552    15569      +17     
==========================================
- Hits        14661    14654       -7     
- Misses        891      915      +24     

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[dennisbader]

Very cool feature, thanks a lot @madtoinou 🚀 Looks like we're close already to merge :)

I had a couple of suggestions regarding the example and the export logic, where I see potential to simplify things further.

[dennisbader]

Looks great now, thanks a lot for changes 🚀 Love this PR 😍