[Feature] Support Lite-HRNet

[Junjun2016]

Motivation

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

Modification

• [x] Add Lite-HRNet backbone and unitests
• [x] Add configs for Lite-HRNet
• [ ] Add benchmarks for Lite-HRNet

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 87.91541% with 40 lines in your changes missing coverage. Please review.

Project coverage is 86.18%. Comparing base (bcafcdd) to head (0f6509a). Report is 365 commits behind head on master.

Files	Patch %	Lines
mmseg/models/backbones/litehrnet.py	87.50%	21 Missing and 19 partials :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #720      +/-   ##
==========================================
+ Coverage   86.09%   86.18%   +0.09%     
==========================================
  Files         108      110       +2     
  Lines        5860     6191     +331     
  Branches      953     1023      +70     
==========================================
+ Hits         5045     5336     +291     
- Misses        630      651      +21     
- Partials      185      204      +19     

Flag	Coverage Δ
unittests	86.17% <87.91%> (+0.09%)	:arrow_up:

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