[Fix]: fix mask AP of small/medium/large

[ZwwWayne]

This PR delete the key bbox in the prediction results when calculating the mask AP of predictions.
When calculating segm AP, if key bbox is in the prediction, the area of instance will be calculated by the box area rather than the mask area, which will not affect the overall mask AP but will lead to a different mask APs of small/medium/large instances. A comparison is as below

model	modification	mask AP	AP50	AP75	APs	APm	APl
Mask R-CNN ms3x (ckpt)	before	0.370	0.584	0.393	0.205	0.402	0.497
	after	0.370	0.584	0.393	0.182	0.395	0.520
Cascade Mask R-CNN 1x (ckpt)	before	0.360	0.564	0.387	0.185	0.387	0.494
	after	0.360	0.564	0.387	0.163	0.382	0.529

[codecov]

Codecov Report

Merging #4898 (6b3a7cd) into master (ca95937) will decrease coverage by 0.73%.
The diff coverage is 37.50%.

@@            Coverage Diff             @@
##           master    #4898      +/-   ##
==========================================
- Coverage   65.61%   64.87%   -0.74%     
==========================================
  Files         255      267      +12     
  Lines       19990    20649     +659     
  Branches     3396     3478      +82     
==========================================
+ Hits        13117    13397     +280     
- Misses       6171     6496     +325     
- Partials      702      756      +54     

Flag	Coverage Δ
unittests	64.85% <37.50%> (-0.73%)	⬇️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmdet/datasets/coco.py	45.84% <30.00%> (-0.47%)	⬇️
...mdet/core/bbox/iou_calculators/iou2d_calculator.py	82.50% <40.90%> (-14.28%)	⬇️
mmdet/models/utils/transformer.py	44.91% <0.00%> (-31.53%)	⬇️
mmdet/models/dense_heads/retina_sepbn_head.py	60.71% <0.00%> (-13.36%)	⬇️
mmdet/models/necks/rfp.py	53.22% <0.00%> (-12.93%)	⬇️
mmdet/models/dense_heads/yolo_head.py	50.92% <0.00%> (-11.03%)	⬇️
mmdet/models/dense_heads/centripetal_head.py	47.82% <0.00%> (-10.42%)	⬇️
mmdet/models/roi_heads/bbox_heads/bbox_head.py	77.94% <0.00%> (-9.99%)	⬇️
mmdet/models/dense_heads/ga_retina_head.py	62.22% <0.00%> (-8.69%)	⬇️
mmdet/models/detectors/rpn.py	58.62% <0.00%> (-8.63%)	⬇️
... and 134 more

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[Johnson-Wang]

This is indeed a breaking change. I think we'd better design a better way to inform the users about the change.

[ZwwWayne]

This is indeed a breaking change. I think we'd better design a better way to inform the users about the change.

Any suggestion? For now, I plan to inform to do that through the compatibility documentation and the changelog.

[Johnson-Wang]

How about adding a reminder in the logger?

[ZwwWayne]

How about adding a reminder in the logger?

Agree, will do that.

[jshilong]

LGTM