Fix bugs in MAP related to iou_type="segm"

[SkafteNicki]

What does this PR do?

Fixes #1144
Fixes #1743

Apparently the MeanAveragePrecision metric has not been tested with ddp=True since its introduction.
There are multiple flaws in this metric and we should probably revamp it completely but for now lets try to get it a bit more stable.

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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[codecov]

Codecov Report

Merging #1763 (31658fc) into master (cf7604d) will decrease coverage by 0%.
The diff coverage is 46%.

Additional details and impacted files

@@          Coverage Diff           @@
##           master   #1763   +/-   ##
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- Coverage      87%     87%   -0%     
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  Files         250     250           
  Lines       14012   14038   +26     
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+ Hits        12255   12266   +11     
- Misses       1757    1772   +15