Feat/time net model

[gdevos010]

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.

TODO

• update doc strings
• add tests
• add proper attribution
• add futures covariate support (this will probably need to be seperate MR)

Fixes #2537.

Summary

Adds the TimesNet mode based on this code
https://github.com/thuml/Time-Series-Library/blob/main/models/TimesNet.py

Other Information

[codecov]

Codecov Report

Attention: Patch coverage is 95.19651% with 11 lines in your changes missing coverage. Please review.

Project coverage is 93.88%. Comparing base (6c3571d) to head (3e5b913).

Files with missing lines	Patch %	Lines
darts/models/forecasting/times_net_model.py	93.20%	7 Missing ⚠️
darts/models/components/embed.py	97.56%	3 Missing ⚠️
darts/models/__init__.py	66.66%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #2538    +/-   ##
========================================
  Coverage   93.88%   93.88%            
========================================
  Files         139      141     +2     
  Lines       14858    15073   +215     
========================================
+ Hits        13949    14152   +203     
- Misses        909      921    +12     

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[gdevos010]

@dennisbader This model is ready for review.