Fix/dlinear and nlinear use_static_cov. with multivariate series

[madtoinou]

Fixes #2036.

Summary

• The assumption on the static covariates dimensions was not accounting for the two possible representation/encoding of the static covariates in multivariate series : either shared across components shape = (1, nb_static_covs) or component-specific shape = (nb_components, nb_static_covs)
• Added corresponding test for the torch models accepting use_static_covariates=True

Other Information

• Added missing type hint in both DLinearModel and NLinearModel
• Fixed small typo in the warning message when torch in not installed

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (36a1c09) 93.85% compared to head (03c3dac) 93.85%.

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Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2070      +/-   ##
==========================================
- Coverage   93.85%   93.85%   -0.01%     
==========================================
  Files         134      134              
  Lines       13123    13111      -12     
==========================================
- Hits        12317    12305      -12     
  Misses        806      806              

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[dennisbader]

Looks good as well, thanks @madtoinou 🚀