revert coco_api to 6031ce1 and add comments

[jshilong]

Revert master to 6031ce1

Motivation

The change of get_cat_ids of coco_api of PR #5243 and PR #5242 may cause bugs of a subclass of CocoDataset such as cityscapes

[codecov]

Codecov Report

Merging #5246 (3483dd6) into master (50b87bc) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #5246   +/-   ##
=======================================
  Coverage   65.29%   65.29%           
=======================================
  Files         276      276           
  Lines       21209    21204    -5     
  Branches     3524     3522    -2     
=======================================
- Hits        13848    13845    -3     
+ Misses       6614     6613    -1     
+ Partials      747      746    -1     

Flag	Coverage Δ
unittests	65.26% <100.00%> (+<0.01%)	⬆️

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Impacted Files	Coverage Δ
mmdet/datasets/coco.py	45.84% <ø> (ø)
mmdet/datasets/api_wrappers/coco_api.py	83.33% <100.00%> (+4.02%)	⬆️

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