[ENH] Implement NBEATS family in v2

[Faakhir30]

WIP

Reference Issues/PRs

What does this implement/fix? Explain your changes.

• NBEATS adapter
• NBEATS model
• NBEATSKAN model
• NBEATSx - https://arxiv.org/abs/2104.05522 - [Suggestion] NBEATS modification for covariate datasets: NBEATSx was released #471

What should a reviewer concentrate their feedback on?

Did you add any tests for the change?

Any other comments?

PR checklist

• The PR title starts with either [ENH], [MNT], [DOC], or [BUG]. [BUG] - bugfix, [MNT] - CI, test framework, [ENH] - adding or improving code, [DOC] - writing or improving documentation or docstrings.
• 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

[codecov]

Codecov Report

❌ Patch coverage is 24.42748% with 99 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@7bcf66c). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...ch_forecasting/models/nbeats/_nbeats_adapter_v2.py	19.35%	50 Missing ⚠️
pytorch_forecasting/models/nbeats/_nbeats_v2.py	22.91%	37 Missing ⚠️
...ytorch_forecasting/models/nbeats/_nbeats_pkg_v2.py	42.85%	12 Missing ⚠️

Additional details and impacted files

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  Coverage        ?   86.66%           
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  Files           ?      178           
  Lines           ?    10296           
  Branches        ?        0           
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  Hits            ?     8923           
  Misses          ?     1373           
  Partials        ?        0           

Flag	Coverage Δ
cpu	86.66% <24.42%> (?)
pytest	86.66% <24.42%> (?)

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[phoeenniixx]

Thanks, I have a suggestion - can you please add one model per PR? this makes it easy to review.

Meaning please add NBEATSKAN and NBEATSx in separate PRs