Fix prefix postfix in nested collection

[SkafteNicki]

What does this PR do?

Fixes #1772

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--1773.org.readthedocs.build/en/1773/

[codecov]

Codecov Report

Merging #1773 (661ff03) into master (3ea6062) will increase coverage by 0%.
The diff coverage is 92%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1773   +/-   ##
======================================
  Coverage      87%     87%           
======================================
  Files         253     253           
  Lines       14146   14164   +18     
======================================
+ Hits        12370   12387   +17     
- Misses       1776    1777    +1