Bug fix - correct for multiple lengths of series in input sequence when adding static covariates to regression models

[eliane-maalouf]

Fixes #1460

Summary

In the original version for the code adding static covariates to regression models, it was assumed that the series are of same length.
This PR corrects this assumption and allows to support cases where the series in the input sequence have different sizes.

Other Information

None

[codecov-commenter]

Codecov Report

Base: 93.95% // Head: 93.91% // Decreases project coverage by -0.03% ⚠️

Coverage data is based on head (f9a9f27) compared to base (ed83ff8).
Patch coverage: 93.54% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1469      +/-   ##
==========================================
- Coverage   93.95%   93.91%   -0.04%     
==========================================
  Files         122      122              
  Lines       10728    10728              
==========================================
- Hits        10079    10075       -4     
- Misses        649      653       +4     

Impacted Files	Coverage Δ
darts/utils/data/tabularization.py	97.67% <93.33%> (-2.33%)	⬇️
darts/models/forecasting/regression_model.py	97.32% <100.00%> (ø)
darts/timeseries.py	91.78% <0.00%> (-0.23%)	⬇️
darts/ad/anomaly_model/filtering_am.py	91.93% <0.00%> (-0.13%)	⬇️
...arts/models/forecasting/torch_forecasting_model.py	89.52% <0.00%> (-0.05%)	⬇️
darts/models/forecasting/block_rnn_model.py	98.24% <0.00%> (-0.04%)	⬇️
darts/models/forecasting/nhits.py	99.27% <0.00%> (-0.01%)	⬇️
darts/datasets/__init__.py	100.00% <0.00%> (ø)

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[hrzn]

LGTM. Just had a small comment about a variable name, and possibly adding one more test. Thanks for coming up with a (non-trivial) fix quickly @eliane-maalouf !

[eliane-maalouf]

I made a modification to the function to control for the order of the static covariates, in case they change among the series in the input. I thought it would be easier to implement the control in code instead of asking the user to make sure a certain order is present in the static covariates.

[hrzn]

I made a modification to the function to control for the order of the static covariates, in case they change among the series in the input. I thought it would be easier to implement the control in code instead of asking the user to make sure a certain order is present in the static covariates.

👍

[hrzn]

LGTM, thanks @eliane-maalouf