backtest can accept a list of callable as metrics functions

[madtoinou]

Fixes #1330.

Summary

Added the support of a sequence of callable as metric function to be applied to measure the distance between the ground-truth and the prediction during backtesting. The model does not have to be retrained to obtain values for each metric function.

Other Information

• The backtest function signature remains identical as before to maintain retro-compatibility with existing tests and code.
• This functionality was not extended to the gridsearch method as optimizing several metrics comes with additional challenges and parameters (weighting of the various metric functions for example). However, such feature could be implemented if necessary.

[codecov-commenter]

Codecov Report

Base: 93.87% // Head: 93.88% // Increases project coverage by +0.00% 🎉

Coverage data is based on head (4ab4b5f) compared to base (06e8e13).
Patch coverage: 100.00% of modified lines in pull request are covered.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #1333   +/-   ##
=======================================
  Coverage   93.87%   93.88%           
=======================================
  Files          78       78           
  Lines        8512     8504    -8     
=======================================
- Hits         7991     7984    -7     
+ Misses        521      520    -1     

Impacted Files	Coverage Δ
darts/models/forecasting/forecasting_model.py	97.01% <100.00%> (+0.33%)	⬆️
darts/timeseries.py	92.30% <0.00%> (-0.06%)	⬇️
...arts/models/forecasting/torch_forecasting_model.py	87.08% <0.00%> (-0.06%)	⬇️
darts/models/forecasting/block_rnn_model.py	98.24% <0.00%> (-0.04%)	⬇️
darts/models/forecasting/nhits.py	99.27% <0.00%> (-0.01%)	⬇️

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[hrzn]

LGTM, thanks @madtoinou ! Just got a small comment about the docstring