Use pycocotools instead of mmpycocotools

[GT9505]

As described in #259 , MMPycocotools is no longer necessary. We use the official pycocotools.

[codecov]

Codecov Report

Merging #263 (1c078f6) into master (a8d4eec) will increase coverage by 0.78%.
The diff coverage is 100.00%.

❗ Current head 1c078f6 differs from pull request most recent head e849ce9. Consider uploading reports for the commit e849ce9 to get more accurate results

@@            Coverage Diff             @@
##           master     #263      +/-   ##
==========================================
+ Coverage   68.64%   69.42%   +0.78%     
==========================================
  Files          90       90              
  Lines        4605     4638      +33     
  Branches      894      894              
==========================================
+ Hits         3161     3220      +59     
+ Misses       1156     1134      -22     
+ Partials      288      284       -4     

Flag	Coverage Δ
unittests	69.36% <100.00%> (+0.78%)	⬆️

Flags with carried forward coverage won't be shown. Click here to find out more.

Impacted Files	Coverage Δ
mmtrack/datasets/imagenet_vid_dataset.py	22.91% <100.00%> (ø)
mmtrack/datasets/parsers/coco_video_parser.py	68.75% <100.00%> (+0.39%)	⬆️
mmtrack/models/reid/linear_reid_head.py	89.85% <0.00%> (+0.14%)	⬆️
mmtrack/models/track_heads/siamese_rpn_head.py	90.52% <0.00%> (+0.15%)	⬆️
mmtrack/models/mot/trackers/sort_tracker.py	74.75% <0.00%> (+0.24%)	⬆️
mmtrack/models/reid/fc_module.py	84.37% <0.00%> (+0.50%)	⬆️
mmtrack/models/mot/tracktor.py	57.35% <0.00%> (+1.29%)	⬆️
mmtrack/models/vid/base.py	40.67% <0.00%> (+2.08%)	⬆️
mmtrack/models/mot/trackers/base_tracker.py	84.73% <0.00%> (+2.19%)	⬆️
mmtrack/models/losses/l2_loss.py	20.63% <0.00%> (+2.60%)	⬆️
... and 5 more

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