speedup static covariates when no casting is needed

[hrzn]

Static covariates of very high dimensionality can cause performance issues due to casting
This PR avoids the casting where/when possible.

[codecov-commenter]

Codecov Report

Merging #1053 (835887c) into master (8b6d1ab) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1053      +/-   ##
==========================================
- Coverage   93.16%   93.15%   -0.01%     
==========================================
  Files          78       78              
  Lines        7948     7937      -11     
==========================================
- Hits         7405     7394      -11     
  Misses        543      543              

Impacted Files	Coverage Δ
darts/timeseries.py	92.15% <100.00%> (-0.05%)	⬇️
...arts/models/forecasting/torch_forecasting_model.py	89.51% <0.00%> (-0.05%)	⬇️
darts/models/forecasting/block_rnn_model.py	98.14% <0.00%> (-0.04%)	⬇️
darts/models/forecasting/nhits.py	99.25% <0.00%> (-0.01%)	⬇️
darts/datasets/__init__.py	100.00% <0.00%> (ø)

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[dennisbader]

Looks good, thanks 👍