[Feature] Support ShuffleNetV2

[Junjun2016]

Motivation

Support more real-time semantic segmentation models. In this PR, we add ShuffleNetV2.

Modification

• [x] Add ShuffleNetV2 backbone and unitests
• [ ] Add configs for ShuffleNetV2
• [ ] Add benchmarks for ShuffleNetV2

BC-breaking (Optional)

Does the modification introduce changes that break the backward-compatibility of the downstream repos? No

[codecov[bot]]

Codecov Report

Attention: Patch coverage is 95.03546% with 7 lines in your changes missing coverage. Please review.

Project coverage is 85.60%. Comparing base (52b4fa5) to head (7513f29). Report is 378 commits behind head on master.

Files with missing lines	Patch %	Lines
mmseg/models/backbones/shufflenet_v2.py	94.61%	3 Missing and 4 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #746      +/-   ##
==========================================
+ Coverage   85.35%   85.60%   +0.25%     
==========================================
  Files         107      109       +2     
  Lines        5823     5988     +165     
  Branches      952      991      +39     
==========================================
+ Hits         4970     5126     +156     
- Misses        672      676       +4     
- Partials      181      186       +5     

Flag	Coverage Δ
unittests	85.58% <95.03%> (+0.25%)	:arrow_up:

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