Fix/backtest multiple series

[hrzn]

Fixes #1516
Fix an issue with backtest() on multiple series.

[codecov-commenter]

Codecov Report

Base: 94.08% // Head: 94.04% // Decreases project coverage by -0.05% ⚠️

Coverage data is based on head (9a383f3) compared to base (7753a6b).
Patch has no changes to coverable lines.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1517      +/-   ##
==========================================
- Coverage   94.08%   94.04%   -0.05%     
==========================================
  Files         122      122              
  Lines       10981    10967      -14     
==========================================
- Hits        10332    10314      -18     
- Misses        649      653       +4     

Impacted Files	Coverage Δ
darts/models/forecasting/forecasting_model.py	97.06% <ø> (ø)
darts/utils/data/tabularization.py	99.27% <0.00%> (-0.73%)	⬇️
darts/timeseries.py	92.14% <0.00%> (-0.23%)	⬇️
darts/ad/anomaly_model/filtering_am.py	91.93% <0.00%> (-0.13%)	⬇️
...arts/models/forecasting/torch_forecasting_model.py	89.52% <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]

LGTM, thanks!

[louisoutin]

Is it possible to have a new release including this change please? :)