refactor: unify on num instead of nb or n

[Borda]

What does this PR do?

Unify the usage of num, nb and n if possible

Before submitting

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• Did you make sure to update the docs?
• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--2090.org.readthedocs.build/en/2090/

[codecov]

Codecov Report

Merging #2090 (773187b) into master (6bf705e) will decrease coverage by 0%.
The diff coverage is 82%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2090   +/-   ##
======================================
- Coverage      87%     87%   -0%     
======================================
  Files         293     293           
  Lines       16304   16299    -5     
======================================
- Hits        14182   14177    -5     
  Misses       2122    2122