fixed non contiguous error when using lstm_layers > 1 on gpu

[dennisbader]

Fixes #722, #730

• fixes TFTModel non contiguous error with lstm_layers > 1 on GPU.
• no performance losses observed

[codecov-commenter]

Codecov Report

Merging #740 (f4e0cb4) into master (adde52a) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master     #740      +/-   ##
==========================================
- Coverage   90.80%   90.80%   -0.01%     
==========================================
  Files          67       67              
  Lines        6741     6740       -1     
==========================================
- Hits         6121     6120       -1     
  Misses        620      620              

Impacted Files	Coverage Δ
darts/models/forecasting/tft_model.py	95.66% <100.00%> (ø)
darts/utils/likelihood_models.py	96.04% <0.00%> (-0.01%)	⬇️

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[hrzn]

Thanks!