Support evaluation on multiple datasets through ConcatDataset

[ZwwWayne]

This PR supports to evaluate concatenated datasets.
Fix #3220, #2907, #3049

[codecov]

Codecov Report

Merging #3522 into master will increase coverage by 1.67%.
The diff coverage is 74.46%.

@@            Coverage Diff             @@
##           master    #3522      +/-   ##
==========================================
+ Coverage   59.04%   60.71%   +1.67%     
==========================================
  Files         205      205              
  Lines       13731    13770      +39     
  Branches     2315     2328      +13     
==========================================
+ Hits         8107     8360     +253     
+ Misses       5248     4990     -258     
- Partials      376      420      +44     

Flag	Coverage Δ
#unittests	60.71% <74.46%> (+1.67%)	⬆️

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Impacted Files	Coverage Δ
mmdet/datasets/custom.py	51.42% <ø> (+15.71%)	⬆️
mmdet/models/roi_heads/bbox_heads/bbox_head.py	85.06% <0.00%> (ø)
mmdet/datasets/builder.py	59.72% <66.66%> (+28.37%)	⬆️
mmdet/models/detectors/base.py	51.14% <66.66%> (ø)
mmdet/datasets/dataset_wrappers.py	84.07% <78.37%> (+3.05%)	⬆️
mmdet/models/roi_heads/test_mixins.py	48.38% <0.00%> (-5.38%)	⬇️
mmdet/datasets/coco.py	45.39% <0.00%> (+33.78%)	⬆️
... and 1 more

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