Narwhalify_from_group_dataframe

[cnhwl]

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 #2730.

Summary

A first draft of refactoring the from_group_dataframe() according to #2661. Since I'm not very familiar with the Narwhals library yet, more help would be appreciated!

[codecov]

Codecov Report

Attention: Patch coverage is 55.00000% with 9 lines in your changes missing coverage. Please review.

Project coverage is 43.90%. Comparing base (a92fa41) to head (198c321).
Report is 2 commits behind head on master.

Files with missing lines	Patch %	Lines
darts/timeseries.py	55.00%	9 Missing ⚠️

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2766      +/-   ##
==========================================
- Coverage   43.95%   43.90%   -0.05%     
==========================================
  Files         223      223              
  Lines       33252    33259       +7     
==========================================
- Hits        14617    14604      -13     
- Misses      18635    18655      +20     

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[dennisbader]

Hi @cnhwl, and thanks a lot for this PR! We were a bit busy over the last week with the release of Darts 0.35.0.
Next week we should be able to review it :)

[dennisbader]

Hi @cnhwl and thanks for this PR, it's a very good start 🚀

I oushed some updates for this PR:

• adapt from_group_dataframe() logic to work with polars
• add polars to from_group_dataframe() tests

The Narwhals team is current fixing an issue related to parallelization using polars DFs wrapped by narwhals (see here). Our tests are expected to fail until then.

I will also have to test how this PR affects the performance of the current from_group_dataframe(). But I'm not too worried.

[cnhwl]

Hi @cnhwl and thanks for this PR, it's a very good start 🚀

I made some updates locally to finalize the PR:

• adapt from_group_dataframe() logic to work with polars
• add polars to from_group_dataframe() tests

I would like to push these changes into your branch. It seems like I don't have permission to do so. Could you grant me access to your forked Darts repository?

Of course! @dennisbader I have been added you as a collaborator on my fork repository 🤝.