multiple TimeSeries for historical_forecasts(), residuals() and backtest() + automatic start

[dumjax]

Summary

Add the possibility to historical forecast, backtest and compute residuals on multiple time series.

Also automatically detects the starting point if start is not specified

[maximilianreimer]

Thanks for the great work! Is there a apporximate time line for when this is going to be part of a realease?

[codecov-commenter]

Codecov Report

Attention: Patch coverage is 96.42857% with 7 lines in your changes missing coverage. Please review.

Project coverage is 93.81%. Comparing base (0d924c8) to head (b1c6291).
Report is 359 commits behind head on master.

Files with missing lines	Patch %	Lines
darts/utils/utils.py	88.00%	3 Missing ⚠️
darts/models/forecasting/forecasting_model.py	98.43%	2 Missing ⚠️
...arts/models/forecasting/torch_forecasting_model.py	90.90%	2 Missing ⚠️

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #1318    +/-   ##
========================================
  Coverage   93.81%   93.81%            
========================================
  Files          94       94            
  Lines        9436     9544   +108     
========================================
+ Hits         8852     8954   +102     
- Misses        584      590     +6     

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[hrzn]

Looks great @dumjax, I only put minor comments.
Maybe the main one is a proposal to use a generator instead of eagerly filling up a list. I think this could scale better and avoid blocking calls until all results are computed. However, I think it could be done in a later PR, as this could maybe be done in other parts of Darts as well.
The second fairly important comment would be managing the progress tracking of the outer loop.

[hrzn]

I usually find it a bit clearer to write Optional[int] instead of Union[int, None].

[hrzn]

Same comments as for historical_forecasts() docstring

[hrzn]

Nice one @dumjax 👏
I have made a few changes directly in your branch. I think we can merge...