Mean Average Precision

[gucifer]

Fixes #{520}

Description: Created a COCO Style Implementation for mAP metric. Both of them are calculated at once since both require similar calculation and have similar use case. The overall implementation is complicated so any advice to simplify it is appreciated. Check list:

• [x] New tests are added (if a new feature is added)
• [x] New doc strings: description and/or example code are in RST format
• [x] Documentation is updated (if required)

[sdesrozis]

We already discussed a plan one week ago :

(1) remove mAR, use one max_det (2) do a comparison with the original pycocotools implementation (3) do an api based on preds/gts of images, it means mostly move pieces of code (4) optimize the code to save space/cpu, it means change the data structures

The first thing you had to do was having a test, yet I don't see any test.

[sdesrozis]

@gucifer It misses tests, from unitary to ddp.

[sdesrozis]

I move to draft instead of PR. A lot of work remaining. Maybe it could worth to discuss, implement and test on our side to create a new clean PR. We will see after GSoC.