Feat/improve zero length datasets

[hrzn]

• Solves RNN Residuals #1062 (bug in min_train_length for RNNModel)
• Improves error messages.
• Introduces retrain option in residuals() method

[codecov-commenter]

Codecov Report

Merging #1066 (6f89562) into master (5ceac68) will decrease coverage by 0.06%.
The diff coverage is 68.42%.

@@            Coverage Diff             @@
##           master    #1066      +/-   ##
==========================================
- Coverage   93.76%   93.69%   -0.07%     
==========================================
  Files          80       80              
  Lines        8207     8213       +6     
==========================================
  Hits         7695     7695              
- Misses        512      518       +6     

Impacted Files	Coverage Δ
darts/models/forecasting/forecasting_model.py	96.36% <ø> (ø)
...arts/models/forecasting/torch_forecasting_model.py	88.69% <62.50%> (-0.91%)	⬇️
darts/models/forecasting/rnn_model.py	97.64% <100.00%> (+0.08%)	⬆️
darts/timeseries.py	92.23% <0.00%> (-0.07%)	⬇️
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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