Fix how prefix/posfix works in MultitaskWrapper

[SkafteNicki]

What does this PR do?

Fixes #2656
This should much more closely match how the users expects to these arguments to work.

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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--2722.org.readthedocs.build/en/2722/

[codecov]

Codecov Report

Attention: Patch coverage is 9.09091% with 20 lines in your changes missing coverage. Please review.

Project coverage is 37%. Comparing base (144f6d6) to head (7fc6974).
Report is 1 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #2722     +/-   ##
========================================
- Coverage      69%     37%    -32%     
========================================
  Files         329     329             
  Lines       18064   18065      +1     
========================================
- Hits        12492    6644   -5848     
- Misses       5572   11421   +5849