Support DMNet

[Junjun2016]

Support new decode_head DMHead proposed in DMNet ICCV'19.

[codecov]

Codecov Report

Merging #313 (c51eb19) into master (e3f6f65) will increase coverage by 0.11%.
The diff coverage is 93.84%.

@@            Coverage Diff             @@
##           master     #313      +/-   ##
==========================================
+ Coverage   85.69%   85.80%   +0.11%     
==========================================
  Files          91       92       +1     
  Lines        4487     4552      +65     
  Branches      716      724       +8     
==========================================
+ Hits         3845     3906      +61     
- Misses        497      499       +2     
- Partials      145      147       +2     

Flag	Coverage Δ
unittests	85.80% <93.84%> (+0.11%)	⬆️

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

Impacted Files	Coverage Δ
mmseg/models/decode_heads/dm_head.py	93.75% <93.75%> (ø)
mmseg/models/decode_heads/__init__.py	100.00% <100.00%> (ø)

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