Fix classwise computation in IoU metric

[SkafteNicki]

What does this PR do?

Fixes #1921
Became a larger refactor of this set of metric than first intended:

• Some of the logic needed to be reworked to fix the issue
• Move all testing to a single file because the metrics are so similar

Before submitting

• Was this discussed/agreed via a Github issue? (no need for typos and docs improvements)
• Did you read the contributor guideline, Pull Request section?
• Did you make sure to update the docs?
• Did you write any new necessary tests?

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📚 Documentation preview 📚: https://torchmetrics--1924.org.readthedocs.build/en/1924/

[Borda]

@SkafteNicki how is it going here? 🐰

[codecov]

Codecov Report

Merging #1924 (22b31af) into master (cc6f6cc) will decrease coverage by 48%.
The diff coverage is 100%.

Additional details and impacted files

@@           Coverage Diff            @@
##           master   #1924     +/-   ##
========================================
- Coverage      87%     38%    -48%     
========================================
  Files         275     275             
  Lines       15664   15650     -14     
========================================
- Hits        13563    5980   -7583     
- Misses       2101    9670   +7569     

[Borda]

@SkafteNicki seems we would need to find around for the oldest PyTorch:
ImportError: cannot import name 'complete_box_iou' from 'torchvision.ops'
Do you know from which version this function is present?