[ENH] Informer Implementation

[Ankit-1204]

Description

This PR tries to implement Informer Transformer, implementation is taken from thuml

Mentioned at sktime #4939

Checklist

• Linked issues (if existing)
• Amended changelog for large changes (and added myself there as contributor)
• Added/modified tests
• Used pre-commit hooks when committing to ensure that code is compliant with hooks. Install hooks with pre-commit install.
  To run hooks independent of commit, execute pre-commit run --all-files

Make sure to have fun coding!

[Ankit-1204]

Hello, this was my first time implementing for pytorch-forecasting, i am currently looking into https://pytorch-forecasting.readthedocs.io/en/stable/tutorials/building.html , for some guidance.
I haven't added much as of now because i had some doubts as implementing models for pytorch-forecasting is somewhat different from sktime.
I gather that we're expected to use TimeSeriesDataSet and from_dataset() .

I'll look into other implementation for better understanding for now.

[codecov]

Codecov Report

Attention: Patch coverage is 0% with 314 lines in your changes missing coverage. Please review.

Please upload report for BASE (main@e87230b). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
pytorch_forecasting/models/informer/sub_modules.py	0.00%	261 Missing ⚠️
pytorch_forecasting/models/informer/_informer.py	0.00%	53 Missing ⚠️

Additional details and impacted files

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  Coverage        ?   81.90%           
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  Files           ?       48           
  Lines           ?     5615           
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Flag	Coverage Δ
cpu	81.90% <0.00%> (?)
pytest	81.90% <0.00%> (?)

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