Use MMCV registry

[GT9505]

This PR supported to use MMCV registry (#760) like MMDet did (#5059).

[codecov]

Codecov Report

Merging #220 (7cafe1e) into master (b3eedb1) will increase coverage by 0.10%.
The diff coverage is 95.65%.

❗ Current head 7cafe1e differs from pull request most recent head 48d3be3. Consider uploading reports for the commit 48d3be3 to get more accurate results

@@            Coverage Diff             @@
##           master     #220      +/-   ##
==========================================
+ Coverage   69.96%   70.07%   +0.10%     
==========================================
  Files          87       87              
  Lines        4402     4394       -8     
  Branches      863      860       -3     
==========================================
- Hits         3080     3079       -1     
+ Misses       1037     1031       -6     
+ Partials      285      284       -1     

Flag	Coverage Δ
unittests	70.00% <95.65%> (+0.12%)	⬆️

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

Impacted Files	Coverage Δ
mmtrack/models/__init__.py	100.00% <ø> (ø)
mmtrack/models/builder.py	89.47% <92.30%> (+20.72%)	⬆️
mmtrack/models/mot/deep_sort.py	60.37% <100.00%> (+0.76%)	⬆️
mmtrack/models/mot/tracktor.py	62.68% <100.00%> (+0.56%)	⬆️
mmtrack/models/vid/dff.py	68.88% <100.00%> (+0.34%)	⬆️
mmtrack/models/vid/fgfa.py	66.12% <100.00%> (+0.27%)	⬆️
mmtrack/models/vid/selsa.py	67.59% <100.00%> (+0.30%)	⬆️
mmtrack/datasets/pipelines/transforms.py	87.81% <0.00%> (-0.32%)	⬇️

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