replaced new LinearLR scheduler (only available with torch >= 1.10.0)…

[dennisbader]

Fixes #915

Summary

• replaced LinearLR tests (only available for torch>=1.10.0) scheduler with StepLR (available from torch>=1.8.0)
• moved save_hyperparameters() repetition into parent class PLForecastingModule

[codecov-commenter]

Codecov Report

Merging #928 (18cadc1) into master (0d1a208) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #928      +/-   ##
==========================================
- Coverage   91.70%   91.70%   -0.01%     
==========================================
  Files          74       74              
  Lines        7404     7398       -6     
==========================================
- Hits         6790     6784       -6     
  Misses        614      614              

Impacted Files	Coverage Δ
darts/models/forecasting/block_rnn_model.py	98.11% <ø> (-0.04%)	⬇️
darts/models/forecasting/nbeats.py	99.26% <ø> (-0.01%)	⬇️
darts/models/forecasting/nhits.py	98.42% <ø> (-0.02%)	⬇️
darts/models/forecasting/rnn_model.py	97.43% <ø> (-0.04%)	⬇️
darts/models/forecasting/tcn_model.py	96.80% <ø> (-0.04%)	⬇️
darts/models/forecasting/tft_model.py	96.79% <ø> (-0.02%)	⬇️
darts/models/forecasting/transformer_model.py	100.00% <ø> (ø)
darts/models/forecasting/pl_forecasting_module.py	93.07% <100.00%> (+0.03%)	⬆️

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