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?

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 🙃

---

📚 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 89 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