fixed an issue using fit_transform() with reconciliators

[hrzn]

Fixes #1035

[codecov-commenter]

Codecov Report

Merging #1165 (85cab08) into master (f953005) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1165      +/-   ##
==========================================
- Coverage   93.65%   93.64%   -0.01%     
==========================================
  Files          80       80              
  Lines        8295     8282      -13     
==========================================
- Hits         7769     7756      -13     
  Misses        526      526              

Impacted Files	Coverage Δ
...arts/dataprocessing/transformers/reconciliation.py	99.01% <100.00%> (ø)
darts/timeseries.py	92.23% <0.00%> (-0.07%)	⬇️
...arts/models/forecasting/torch_forecasting_model.py	87.45% <0.00%> (-0.05%)	⬇️
darts/models/forecasting/block_rnn_model.py	98.24% <0.00%> (-0.04%)	⬇️
darts/models/forecasting/nhits.py	98.55% <0.00%> (-0.02%)	⬇️
darts/datasets/__init__.py	100.00% <0.00%> (ø)

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