bump: testing with next PyTorch 2.6

[Borda]

What does this PR do?

adding testing for the next version with PT 2.6

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

PR review

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If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

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📚 Documentation preview 📚: https://torchmetrics--2836.org.readthedocs.build/en/2836/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69%. Comparing base (7c16833) to head (ea62c98).
Report is 1 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2836   +/-   ##
======================================
  Coverage      69%     69%           
======================================
  Files         346     346           
  Lines       19128   19130    +2     
======================================
+ Hits        13226   13228    +2     
  Misses       5902    5902           

[Borda]

seems like needed torch* packages have not been cut to testing yet...