Fix expected box format by pycoco

[SkafteNicki]

What does this PR do?

Fixes #1908

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--1913.org.readthedocs.build/en/1913/

[codecov]

Codecov Report

Merging #1913 (300a37a) into master (a448ad3) will decrease coverage by 47%.
The diff coverage is 100%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1913     +/-   ##
========================================
- Coverage      85%     38%    -47%     
========================================
  Files         262     262             
  Lines       15054   15054             
========================================
- Hits        12803    5788   -7015     
- Misses       2251    9266   +7015