Warning on many detections in MeanAveragePrecision

[SkafteNicki]

What does this PR do?

Fixes #1974

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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--1978.org.readthedocs.build/en/1978/

[codecov]

Codecov Report

Merging #1978 (01bb81b) into master (e942f69) will decrease coverage by 48%.
Report is 1 commits behind head on master.
The diff coverage is 100%.

❗ Current head 01bb81b differs from pull request most recent head ac3b757. Consider uploading reports for the commit ac3b757 to get more accurate results

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1978     +/-   ##
========================================
- Coverage      86%     39%    -48%     
========================================
  Files         266     266             
  Lines       15177   15185      +8     
========================================
- Hits        13123    5857   -7266     
- Misses       2054    9328   +7274