[Refactor] Use MMCV Model Registry

[dreamerlin]

This PR Use MMCV Model Registry and remove registry file.

[codecov]

Codecov Report

Merging #843 (c921088) into master (9bfdbbe) will decrease coverage by 1.94%.
The diff coverage is 29.24%.

❗ Current head c921088 differs from pull request most recent head 8f54c75. Consider uploading reports for the commit 8f54c75 to get more accurate results

@@            Coverage Diff             @@
##           master     #843      +/-   ##
==========================================
- Coverage   85.21%   83.27%   -1.95%     
==========================================
  Files         131      129       -2     
  Lines        9415     9421       +6     
  Branches     1612     1612              
==========================================
- Hits         8023     7845     -178     
- Misses        986     1185     +199     
+ Partials      406      391      -15     

Flag	Coverage Δ
unittests	83.27% <29.24%> (-1.95%)	⬇️

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

Impacted Files	Coverage Δ
mmaction/apis/test.py	12.37% <5.06%> (-59.54%)	⬇️
mmaction/core/evaluation/eval_hooks.py	11.76% <7.50%> (-72.62%)	⬇️
mmaction/datasets/builder.py	44.44% <80.00%> (+0.25%)	⬆️
mmaction/models/builder.py	58.33% <94.44%> (+7.26%)	⬆️
mmaction/datasets/__init__.py	100.00% <100.00%> (ø)
mmaction/datasets/activitynet_dataset.py	92.72% <100.00%> (ø)
mmaction/datasets/audio_dataset.py	73.68% <100.00%> (ø)
mmaction/datasets/audio_feature_dataset.py	73.68% <100.00%> (ø)
mmaction/datasets/audio_visual_dataset.py	76.74% <100.00%> (ø)
mmaction/datasets/ava_dataset.py	90.60% <100.00%> (ø)
... and 51 more

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