Fix/bug in ensemble models to support underlying stochastic models

[eliane-maalouf]

Fixes #1390 .

Summary

• added _is_probabilistic() to some of the baseline models that do not have it
• added _is_probabilistic() to ensemble classes
• added a basic unitest for the case

[codecov-commenter]

Codecov Report

Base: 93.67% // Head: 93.67% // Decreases project coverage by -0.00% ⚠️

Coverage data is based on head (0adeab2) compared to base (cdf2b44).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1423      +/-   ##
==========================================
- Coverage   93.67%   93.67%   -0.01%     
==========================================
  Files          94       94              
  Lines        9398     9387      -11     
==========================================
- Hits         8804     8793      -11     
  Misses        594      594              

Impacted Files	Coverage Δ
darts/models/forecasting/ensemble_model.py	92.00% <100.00%> (+0.33%)	⬆️
darts/timeseries.py	91.71% <0.00%> (-0.06%)	⬇️
...arts/models/forecasting/torch_forecasting_model.py	89.50% <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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[dennisbader]

Thanks @eliane-maalouf , looks good to me :)
Only added some minor suggestions.

[eliane-maalouf]

thanks @dennisbader for your review. It should be ok now :)

[dennisbader]

this one can be removed as well :) after that all looks good!

[eliane-maalouf]

;) missed this one, thanks

[dennisbader]

LGTM, thanks!