CI: 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

Anyone in the community is free to review the PR once the tests have passed.
If we didn't discuss your PR in Github issues there's a high chance it will not be merged.

Did you have fun?

Make sure you had fun coding 🙃

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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 (962af66) to head (c6e2224).

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2836   +/-   ##
======================================
  Coverage      69%     69%           
======================================
  Files         346     346           
  Lines       19129   19129           
======================================
  Hits        13227   13227           
  Misses       5902    5902