[Refactor]: Inherits mmcv registry

[ZwwWayne]

This PR modifies the registry of models. It changes to inherit from MMCV registry, thus in the future OpenMMLab repos like MMDet and MMSeg could directly use the backbones supported in MMCls.

Related PR: open-mmlab/mmdetection#5059, open-mmlab/mmsegmentation#515

[codecov]

Codecov Report

Merging #252 (437e286) into master (8c90a87) will increase coverage by 0.04%.
The diff coverage is 93.33%.

@@            Coverage Diff             @@
##           master     #252      +/-   ##
==========================================
+ Coverage   72.83%   72.88%   +0.04%     
==========================================
  Files          88       88              
  Lines        4455     4451       -4     
  Branches      782      780       -2     
==========================================
- Hits         3245     3244       -1     
+ Misses       1109     1107       -2     
+ Partials      101      100       -1     

Flag	Coverage Δ
unittests	72.88% <93.33%> (+0.04%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/models/builder.py	94.44% <93.33%> (+12.62%)	⬆️

---

Continue to review full report at Codecov.

Legend - Click here to learn more
Δ = absolute <relative> (impact), ø = not affected, ? = missing data
Powered by Codecov. Last update 8c90a87...437e286. Read the comment docs.

[ycxioooong]

Thanks for this PR. LGTM.