fix MixedCovTorchModels multi TS predictions with n<ocl

[dennisbader]

Checklist before merging this PR:

• Mentioned all issues that this PR fixes or addresses.
• Summarized the updates of this PR under Summary.
• Added an entry under Unreleased in the Changelog.

Summary

• fixes a bug when calling predict() with a MixedCovariatesTorchForecastingModel (e.g. TiDE, N/DLinear, ...) n<output_chunk_length and a list of series with length len(series) < n, where the predictions did not return the correct number of series.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.77%. Comparing base (2430903) to head (6a98960).

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2374      +/-   ##
==========================================
- Coverage   93.79%   93.77%   -0.02%     
==========================================
  Files         138      138              
  Lines       14288    14271      -17     
==========================================
- Hits        13401    13383      -18     
- Misses        887      888       +1     

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[madtoinou]

LGTM 🚀