Add metric items for COCO

[xvjiarui]

This PR added metric_items option for COCO evaluation.
It also fixed #3468

[codecov]

Codecov Report

Merging #3497 into master will decrease coverage by 0.13%.
The diff coverage is 0.00%.

@@            Coverage Diff             @@
##           master    #3497      +/-   ##
==========================================
- Coverage   58.79%   58.65%   -0.14%     
==========================================
  Files         205      205              
  Lines       13609    13621      +12     
  Branches     2302     2310       +8     
==========================================
- Hits         8002     7990      -12     
- Misses       5232     5255      +23     
- Partials      375      376       +1     

Flag	Coverage Δ
#unittests	58.65% <0.00%> (-0.14%)	⬇️

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Impacted Files	Coverage Δ
mmdet/datasets/coco.py	11.60% <0.00%> (-0.50%)	⬇️
mmdet/models/roi_heads/mask_scoring_roi_head.py	81.57% <0.00%> (-18.43%)	⬇️
mmdet/models/roi_heads/mask_heads/maskiou_head.py	92.30% <0.00%> (-5.50%)	⬇️

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[Johnson-Wang]

Better add assert isinstance(metric_items, str)

[zhjw0927]

I'm still not sure how to use it?
If I evaluate bbox in coco, run python3 tool/test.py configs/**.py work_dirs/**.pth --eval bbox
then?

[xvjiarui]

If you would like to specify metric_items, you may pass in --eval-options metric_items=mAP_50