Fix getitem for metric collection when prefix/postfix is set

[SkafteNicki]

What does this PR do?

Fixes #2399

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• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--2430.org.readthedocs.build/en/2430/

[codecov]

Codecov Report

Merging #2430 (60fe120) into master (e5b1135) will decrease coverage by 0%.
Report is 1 commits behind head on master.
The diff coverage is 0%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2430   +/-   ##
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- Coverage      69%     69%   -0%     
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  Files         307     307           
  Lines       17396   17400    +4     
======================================
  Hits        11981   11981           
- Misses       5415    5419    +4     

[Borda]

seems like PT 1.13 does not like it :)

FAILED unittests/bases/test_collections.py::test_metric_collection_prefix_postfix_args[None-None] - AttributeError: 'str' object has no attribute 'removeprefix'
FAILED unittests/bases/test_collections.py::test_metric_collection_prefix_postfix_args[prefix_-None] - AttributeError: 'str' object has no attribute 'removeprefix'
FAILED unittests/bases/test_collections.py::test_metric_collection_prefix_postfix_args[None-_postfix] - AttributeError: 'str' object has no attribute 'removeprefix'
FAILED unittests/bases/test_collections.py::test_metric_collection_prefix_postfix_args[prefix_-_postfix] - AttributeError: 'str' object has no attribute 'removeprefix'