Fix: estimator getter and lagged_label_name

[madtoinou]

Fixes #2217.

Summary

• Fixed bug in get_multioutput_estimator() where the incorrect estimator could be returned
• Added an attribute to store the lagged label, each one corresponding to an estimator

Other Information

• Added tests where the estimator are over-fitted on crafted examples to check if the appropriate estimator is returned by the getter.

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 93.33333% with 1 lines in your changes are missing coverage. Please review.

Project coverage is 93.97%. Comparing base (ccd0d42) to head (ad2e32a). Report is 1 commits behind head on master.

Files	Patch %	Lines
darts/models/forecasting/regression_model.py	93.33%	1 Missing :warning:

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2246      +/-   ##
==========================================
+ Coverage   93.95%   93.97%   +0.01%     
==========================================
  Files         135      135              
  Lines       13501    13511      +10     
==========================================
+ Hits        12685    12697      +12     
+ Misses        816      814       -2     

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