Fixed MAP metric when using custom list of thresholds

[SkafteNicki]

What does this PR do?

Fixes #994

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[codecov]

Codecov Report

Merging #995 (ecb6f2e) into master (3a141ae) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff          @@
##           master   #995   +/-   ##
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  Coverage      94%    95%           
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  Files         179    179           
  Lines        7640   7644    +4     
=====================================
+ Hits         7218   7242   +24     
+ Misses        422    402   -20     

[ekmungi]

I have a question about this suggested fix:

I am wondering, how can I get the mAP for a custom IOU threshold = 0.25 for example? The resulting dict from mean_ap.py from the fix still gives the following format. Is the mAP calculated internally for a custom IOU (for example = 0.25)?

{'map': , 'map_50': , 'map_75': , 'map_large': , 'map_medium': , 'map_per_class': , 'map_small': , 'mar_1': , 'mar_10': , 'mar_100': , 'mar_100_per_class': , 'mar_large': , 'mar_medium': , 'mar_small': }