[Fix] fix cocoapi in parser

[stacezhou]

• [Fix] fix cocoapi in parser as what mmdetection have done #4399
• fix a bug

[codecov]

Codecov Report

Merging #218 (8adfbc5) into master (b3eedb1) will increase coverage by 0.10%.
The diff coverage is 88.46%.

@@            Coverage Diff             @@
##           master     #218      +/-   ##
==========================================
+ Coverage   69.96%   70.07%   +0.10%     
==========================================
  Files          87       88       +1     
  Lines        4402     4427      +25     
  Branches      863      864       +1     
==========================================
+ Hits         3080     3102      +22     
- Misses       1037     1039       +2     
- Partials      285      286       +1     

Flag	Coverage Δ
unittests	70.00% <88.46%> (+0.12%)	⬆️

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Impacted Files	Coverage Δ
mmtrack/datasets/parsers/coco_api.py	87.50% <87.50%> (ø)
mmtrack/datasets/parsers/coco_video_parser.py	68.75% <100.00%> (+0.39%)	⬆️

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[GT9505]

Hi~, can you fix the lint error firstly?
Besides, we don't need copy the COCO class from mmdet. We can directly import it (from mmdet.datasets.api_wrappers import COCO).

[GT9505]

Please also modify the mmpycocotools to pycocotools in requirements/runtime.txt, just like #4399 did.

[GT9505]

This PR is fixed in #263 , therefore, I will close this PR

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