Fix/quantile-multiple-ts-prediction

[gnwhr]

Summary

• Now instances of RegressionModel capable of fitting quantiles correctly work with multi TimeSeries input
• Registered RegressionModel instances capable of fitting quantiles to more tests

[codecov-commenter]

Codecov Report

Merging #853 (27007e6) into master (a7abedf) will increase coverage by 0.04%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #853      +/-   ##
==========================================
+ Coverage   91.40%   91.45%   +0.04%     
==========================================
  Files          70       71       +1     
  Lines        7106     7147      +41     
==========================================
+ Hits         6495     6536      +41     
  Misses        611      611              

Impacted Files	Coverage Δ
darts/models/forecasting/gradient_boosted_model.py	100.00% <100.00%> (ø)
...arts/models/forecasting/linear_regression_model.py	94.73% <100.00%> (-0.58%)	⬇️
darts/models/forecasting/regression_model.py	97.11% <100.00%> (+0.34%)	⬆️
darts/utils/losses.py	100.00% <100.00%> (ø)

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[hrzn]

LGTM, thanks 👍