fix failing torch test for torchmetrics when multi output

[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 failing unit tests for torch metrics for multivariate forecasting (since torchmetrics v1.5.0)

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 93.87%. Comparing base (264e6b3) to head (d5f8d68).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2573      +/-   ##
==========================================
- Coverage   93.88%   93.87%   -0.02%     
==========================================
  Files         139      139              
  Lines       14858    14848      -10     
==========================================
- Hits        13949    13938      -11     
- Misses        909      910       +1     

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