[Feature] Support HourglassNet

[Junjun2016]

Motivation

Support more backbones for semantic segmentation. In this PR, we add HourglassNet, which may train from scratch without ImageNet pre-train.

Modification

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

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 97.01493% with 2 lines in your changes missing coverage. Please review.

Project coverage is 85.41%. Comparing base (4d34581) to head (0f13d01). Report is 387 commits behind head on master.

Files	Patch %	Lines
mmseg/models/backbones/hourglass.py	96.96%	1 Missing and 1 partial :warning:

Additional details and impacted files

@@            Coverage Diff             @@
##           master     #724      +/-   ##
==========================================
+ Coverage   85.28%   85.41%   +0.13%     
==========================================
  Files         107      108       +1     
  Lines        5817     5884      +67     
  Branches      952      963      +11     
==========================================
+ Hits         4961     5026      +65     
- Misses        673      674       +1     
- Partials      183      184       +1     

Flag	Coverage Δ
unittests	85.40% <97.01%> (+0.13%)	:arrow_up:

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

:umbrella: View full report in Codecov by Sentry.
:loudspeaker: Have feedback on the report? Share it here.