let lightning handle moving preds to cpu

[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.

Fixes #2828.

Summary

• Fixed an issue with TorchForecastingModel.predict() on Windows machines, where tensor movement from GPU to CPU was not synchronized.
  • Instead of moving to CPU in LightningModule.predict_step(), we let Lightning handle it under the hood.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 95.18%. Comparing base (07e549d) to head (800545f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #2829      +/-   ##
==========================================
- Coverage   95.19%   95.18%   -0.01%     
==========================================
  Files         145      145              
  Lines       15126    15124       -2     
==========================================
- Hits        14399    14396       -3     
- Misses        727      728       +1     

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[madtoinou]

Just a minor comment, otherwise it looks good to me.