add CompatibleCheckHook

[jshilong]

Check whether the num_classes in head matches the number of CLASSSES in dataset.

[codecov]

Codecov Report

Merging #4508 (e700fe0) into master (247785c) will decrease coverage by 0.08%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #4508      +/-   ##
==========================================
- Coverage   62.65%   62.57%   -0.09%     
==========================================
  Files         245      245              
  Lines       18763    18783      +20     
  Branches     3138     3142       +4     
==========================================
- Hits        11756    11753       -3     
- Misses       6432     6456      +24     
+ Partials      575      574       -1     

Flag	Coverage Δ
unittests	62.53% <100.00%> (-0.08%)	⬇️

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Impacted Files	Coverage Δ
mmdet/datasets/utils.py	83.78% <100.00%> (+15.36%)	⬆️
mmdet/models/roi_heads/mask_scoring_roi_head.py	55.35% <0.00%> (-32.15%)	⬇️
mmdet/models/roi_heads/mask_heads/maskiou_head.py	94.62% <0.00%> (-5.38%)	⬇️
mmdet/core/mask/structures.py	87.87% <0.00%> (ø)
mmdet/models/dense_heads/fcos_head.py	60.97% <0.00%> (+0.38%)	⬆️

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