Deprecate num_outputs in R2 because it is no longer needed

[SkafteNicki]

What does this PR do?

Fixes #2703

Passes locally, but lets see if the full test suit passes

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?

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📚 Documentation preview 📚: https://torchmetrics--2705.org.readthedocs.build/en/2705/

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 69%. Comparing base (708f11d) to head (91722ed).
Report is 1 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2705   +/-   ##
======================================
  Coverage      69%     69%           
======================================
  Files         329     329           
  Lines       18070   18071    +1     
======================================
+ Hits        12496   12497    +1     
  Misses       5574    5574