[Enhancement] Support hrnet frozen stage

[sshuair]

Motivation

Add new feature to support HRNet frozen stage like ResNet.

Modification

• Add new parameter frozen_stages for HRNet.
• Add new method _freeze_stages for HRNet.
• Add new test function test_hrnet_backbone to test the frozen_stages.

BC-breaking (Optional)

No.

Use cases (Optional)

1. use the frozen_stages from code

   model = HRNet(extra, frozen_stages=1) # frozen the first stage

2. use the frozen_stages from config

   _base_ = './fcn_hr18_512x512_20k_voc12aug.py'
   model = dict(
       pretrained='open-mmlab://msra/hrnetv2_w18_small',
       backbone=dict(
           frozen_stages=1,
           extra=dict(
               stage1=dict(num_blocks=(2, )),
               stage2=dict(num_blocks=(2, 2)),
               stage3=dict(num_modules=3, num_blocks=(2, 2, 2)),
               stage4=dict(num_modules=2, num_blocks=(2, 2, 2, 2)))))

Checklist

1. Pre-commit used.
2. The modification is covered by complete unit tests.
3. The modification has no potential influence on downstream projects.
4. No need documents.

[codecov]

Codecov Report

Merging #743 (7fae39c) into master (503ada7) will increase coverage by 0.02%.
The diff coverage is 91.66%.

@@            Coverage Diff             @@
##           master     #743      +/-   ##
==========================================
+ Coverage   85.35%   85.37%   +0.02%     
==========================================
  Files         107      107              
  Lines        5823     5847      +24     
  Branches      952      960       +8     
==========================================
+ Hits         4970     4992      +22     
- Misses        672      673       +1     
- Partials      181      182       +1     

Flag	Coverage Δ
unittests	85.36% <91.66%> (+0.02%)	⬆️

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

Impacted Files	Coverage Δ
mmseg/models/backbones/hrnet.py	83.33% <91.66%> (+0.87%)	⬆️

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[Junjun2016]

Hi @sshuair
Thanks for your nice PR!

[Junjun2016]

It seems that the unittest of HRNet are missing in our original codebase.
We appreciate it a lot if you could add more unittests for HRNet.

[sshuair]

It seems that the unittest of HRNet are missing in our original codebase.
We appreciate it a lot if you could add more unittests for HRNet.

@Junjun2016 I will add more HRNet unitest when I am free, but now I am busy at some work.
So, could you merge this PR to the master branch first.

[xvjiarui]

Move this line inside train() like resnet.

[sshuair]

@xvjiarui I did not find any difference between the resnet and hrnet about self._freeze_stages(). Could you give more information?

[xvjiarui]

Sorry. My bad, I forgot to expand the collapsed code lines.