[Feature] Add ResNeSt configs

[LXXXXR]

Hi，

These configs give acc of 80.41 for ResNeSt50 while the original Pytorch code achives 80.61.

Pls kindly take a look and see more details in the doc.

Thanks.

[codecov]

Codecov Report

Merging #332 (094af05) into master (53c0df2) will increase coverage by 0.32%.
The diff coverage is 97.95%.

@@            Coverage Diff             @@
##           master     #332      +/-   ##
==========================================
+ Coverage   73.29%   73.61%   +0.32%     
==========================================
  Files          92       93       +1     
  Lines        4681     4753      +72     
  Branches      793      803      +10     
==========================================
+ Hits         3431     3499      +68     
- Misses       1157     1160       +3     
- Partials       93       94       +1     

Flag	Coverage Δ
unittests	73.61% <97.95%> (+0.32%)	⬆️

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

Impacted Files	Coverage Δ
mmcls/models/heads/stacked_head.py	97.10% <97.10%> (ø)
mmcls/models/backbones/__init__.py	100.00% <100.00%> (ø)
mmcls/models/backbones/mobilenet_v3.py	100.00% <100.00%> (ø)
mmcls/models/heads/__init__.py	100.00% <100.00%> (ø)
mmcls/models/utils/inverted_residual.py	100.00% <100.00%> (ø)
mmcls/models/utils/se_layer.py	90.47% <100.00%> (+1.00%)	⬆️
mmcls/models/utils/augment/identity.py	85.71% <0.00%> (-14.29%)	⬇️

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[ycxioooong]

LGTM