add resnest and vgg backbone

[jin-s13]

The codes are from MMClassification.

[codecov]

Codecov Report

Merging #370 (556768c) into master (b90aa3a) will increase coverage by 0.13%.
The diff coverage is 88.62%.

@@            Coverage Diff             @@
##           master     #370      +/-   ##
==========================================
+ Coverage   81.62%   81.76%   +0.13%     
==========================================
  Files         116      118       +2     
  Lines        7555     7766     +211     
  Branches     1208     1240      +32     
==========================================
+ Hits         6167     6350     +183     
- Misses       1143     1162      +19     
- Partials      245      254       +9     

Flag	Coverage Δ
unittests	81.76% <88.62%> (+0.13%)	⬆️

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Impacted Files	Coverage Δ
mmpose/models/backbones/vgg.py	86.31% <86.31%> (ø)
mmpose/models/backbones/resnest.py	90.35% <90.35%> (ø)
mmpose/models/backbones/__init__.py	100.00% <100.00%> (ø)
mmpose/datasets/pipelines/top_down_transform.py	58.88% <0.00%> (-1.32%)	⬇️

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[innerlee]

Did you run some configs containing them, for the sake of sanity check?

[jin-s13]

Not yet. Will add some related config files and checkpoints later.

[innerlee]

Also update repo's README