tests: cleaning classif.

[Borda]

What does this PR do?

clarify using arguments and adding tests

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.

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📚 Documentation preview 📚: https://torchmetrics--2815.org.readthedocs.build/en/2815/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69%. Comparing base (d52cb48) to head (3d7f057).
Report is 1 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2815   +/-   ##
======================================
  Coverage      69%     69%           
======================================
  Files         344     344           
  Lines       18828   18828           
======================================
  Hits        12975   12975           
  Misses       5853    5853