[ENH] Reformer forecaster added in ptf-v2

[lucifer4073]

Reference Issues/PRs

Closes #2071.

What does this implement/fix? Explain your changes.

Integrates the Reformer forecaster into ptf-v2.

What should a reviewer concentrate their feedback on?

Check if the implementation technique is correct. There are 2 forecasting options long_term_forecast (default) and short_term_forecast.

Did you add any tests for the change?

Appropriate test parameters were added in reformer_pkg_v2.

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

[fkiraly]

Thanks!

There is an incorrect import, please fix

[codecov]

Codecov Report

❌ Patch coverage is 78.29457% with 84 lines in your changes missing coverage. Please review.
⚠️ Please upload report for BASE (main@6b27898). Learn more about missing BASE report.

Files with missing lines	Patch %	Lines
...ecasting/layers/_embeddings/_temporal_embedding.py	23.07%	30 Missing ⚠️
...orecasting/layers/_reformer/_lsh_self_attention.py	80.85%	27 Missing ⚠️
pytorch_forecasting/models/reformer/reformer_v2.py	75.94%	19 Missing ⚠️
pytorch_forecasting/layers/_reformer/_encoder.py	83.72%	7 Missing ⚠️
..._forecasting/layers/_embeddings/_data_embedding.py	95.65%	1 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff           @@
##             main    #2089   +/-   ##
=======================================
  Coverage        ?   86.31%           
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  Files           ?      174           
  Lines           ?    10119           
  Branches        ?        0           
=======================================
  Hits            ?     8734           
  Misses          ?     1385           
  Partials        ?        0           

Flag	Coverage Δ
cpu	86.31% <78.29%> (?)
pytest	86.31% <78.29%> (?)

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[lucifer4073]

I have resolved the import related issues.
Thanks.

[lucifer4073]

Hi @PranavBhatP @phoeenniixx,

Could you please look into this ? It's been a week since I have updated the PR and all tests are passing.
Thanks.