Add Semantic FPN

[xvjiarui]

No description provided.

[hellock]

This PR contains both Semantic FPN and HRFPN. Better to raise two PRs.

[codecov]

Codecov Report

Merging #94 into master will decrease coverage by 0.52%.
The diff coverage is 66.07%.

@@            Coverage Diff             @@
##           master      #94      +/-   ##
==========================================
- Coverage   81.97%   81.44%   -0.53%     
==========================================
  Files          76       79       +3     
  Lines        3467     3579     +112     
  Branches      529      557      +28     
==========================================
+ Hits         2842     2915      +73     
- Misses        522      557      +35     
- Partials      103      107       +4     

Flag	Coverage Δ
#unittests	81.44% <66.07%> (-0.53%)	⬇️

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

Impacted Files	Coverage Δ
mmseg/models/necks/fpn.py	51.89% <51.89%> (ø)
mmseg/models/__init__.py	100.00% <100.00%> (ø)
mmseg/models/decode_heads/__init__.py	100.00% <100.00%> (ø)
mmseg/models/decode_heads/fpn_head.py	100.00% <100.00%> (ø)
mmseg/models/necks/__init__.py	100.00% <100.00%> (ø)
mmseg/models/decode_heads/sep_fcn_head.py	50.00% <0.00%> (-50.00%)	⬇️
mmseg/models/segmentors/encoder_decoder.py	90.00% <0.00%> (+2.66%)	⬆️
... and 1 more

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