Enable semantic segmentation backbone and head

[ethanwharris]

What does this PR do?

Fixes # (issue)

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[codecov]

Codecov Report

Merging #412 (e256ef1) into master (25d6633) will decrease coverage by 0.18%.
The diff coverage is 74.11%.

@@            Coverage Diff             @@
##           master     #412      +/-   ##
==========================================
- Coverage   90.26%   90.08%   -0.19%     
==========================================
  Files         111      112       +1     
  Lines        6957     6999      +42     
==========================================
+ Hits         6280     6305      +25     
- Misses        677      694      +17     

Flag	Coverage Δ
gpu	?
pytest	?
unittests	90.08% <74.11%> (-0.10%)	⬇️

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

Impacted Files	Coverage Δ
flash/core/model.py	78.30% <50.00%> (ø)
flash/image/segmentation/heads.py	68.85% <68.85%> (ø)
flash/image/segmentation/model.py	86.56% <80.00%> (-0.10%)	⬇️
flash/image/segmentation/backbones.py	100.00% <100.00%> (+10.86%)	⬆️
flash/image/backbones.py	78.64% <0.00%> (-1.95%)	⬇️

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[tchaton]

Looks great !