Separate coco from map

[Isalia20]

What does this PR do?

Part of the fix of #2877. This PR decouples the coco backend from the mAP metric. This is part of the fix of 2877 issue because for me to implement functional mAP I would have to duplicate a lot of code unless this class was created. This class will help to duplicate as little code as possible while implementing functional version of mAP. It also makes mAP less heavy with coco stuff and abstracts it away as a coco backend private instance

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?

Did you have fun? yes

Make sure you had fun coding 🙃

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📚 Documentation preview 📚: https://torchmetrics--2884.org.readthedocs.build/en/2884/

[Isalia20]

Huh weird, I thought pre commit checks covered the mypy checks as well. Will fix them

[codecov]

Codecov Report

Attention: Patch coverage is 50.00000% with 99 lines in your changes missing coverage. Please review.

Project coverage is 69%. Comparing base (7bda8f7) to head (b89fe5f).
Report is 1 commits behind head on master.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #2884   +/-   ##
======================================
  Coverage      69%     69%           
======================================
  Files         332     333    +1     
  Lines       18966   18984   +18     
======================================
+ Hits        13055   13103   +48     
+ Misses       5911    5881   -30     

[Isalia20]

I'll wait to get some comments on this first and then proceed with functional part of mAP