Implement pre and postfix for Classwise Wrapper

[relativityhd]

What does this PR do?

Fixes #1867
Adds two new parameter to the ClasswiseWrapper class to change the naming behaviour of it's outputs.

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?

Yessss 🙃

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📚 Documentation preview 📚: https://torchmetrics--1866.org.readthedocs.build/en/1866/

[codecov]

Codecov Report

Merging #1866 (fba600f) into master (8c5fab8) will decrease coverage by 45%.
The diff coverage is 100%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1866     +/-   ##
========================================
- Coverage      85%     40%    -45%     
========================================
  Files         260     260             
  Lines       14908   14918     +10     
========================================
- Hits        12657    5984   -6673     
- Misses       2251    8934   +6683