Fix/ensemble predict with series

[dennisbader]

Fixes #1340

Summary

• fixes several issues in RegressionEnsembleModel, when using only a single target series at fitting time, and passing a new/multiple series to predict()
• removes "static" ìs_single_seriesattribute from ensemble model, and instead recompute it whenever we get a newseries`.
• removes some unnecessary test skips for old regression models torch dependency

[codecov-commenter]

Codecov Report

Base: 93.89% // Head: 93.88% // Decreases project coverage by -0.01% ⚠️

Coverage data is based on head (d32e5d0) compared to base (a0ebdfd).
Patch coverage: 92.85% of modified lines in pull request are covered.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #1357      +/-   ##
==========================================
- Coverage   93.89%   93.88%   -0.02%     
==========================================
  Files          78       78              
  Lines        8522     8508      -14     
==========================================
- Hits         8002     7988      -14     
  Misses        520      520              

Impacted Files	Coverage Δ
darts/models/forecasting/ensemble_model.py	91.66% <83.33%> (-0.65%)	⬇️
...ts/models/forecasting/regression_ensemble_model.py	100.00% <100.00%> (ø)
darts/timeseries.py	92.34% <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!