Add more backbones to semantic segmentation

[ethanwharris]

What does this PR do?

Fixes #369

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[codecov]

Codecov Report

Merging #370 (e62fc9b) into master (447c28e) will decrease coverage by 9.17%.
The diff coverage is 68.18%.

❗ Current head e62fc9b differs from pull request most recent head ee56cd7. Consider uploading reports for the commit ee56cd7 to get more accurate results

@@            Coverage Diff             @@
##           master     #370      +/-   ##
==========================================
- Coverage   88.82%   79.65%   -9.18%     
==========================================
  Files          80       80              
  Lines        4144     4173      +29     
==========================================
- Hits         3681     3324     -357     
- Misses        463      849     +386     

Flag	Coverage Δ
gpu	?
pytest	?
unittests	79.65% <68.18%> (-9.18%)	⬇️

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

Impacted Files	Coverage Δ
flash/image/segmentation/model.py	42.62% <0.00%> (-47.21%)	⬇️
flash/image/backbones.py	67.96% <64.28%> (-18.45%)	⬇️
flash/image/segmentation/backbones.py	79.06% <75.00%> (-20.94%)	⬇️
flash/image/detection/data.py	40.32% <0.00%> (-56.46%)	⬇️
flash/image/segmentation/data.py	36.87% <0.00%> (-51.88%)	⬇️
flash/image/classification/data.py	47.67% <0.00%> (-48.84%)	⬇️
flash/image/detection/model.py	33.33% <0.00%> (-46.67%)	⬇️
flash/image/segmentation/transforms.py	60.00% <0.00%> (-40.00%)	⬇️
flash/image/detection/finetuning.py	62.50% <0.00%> (-37.50%)	⬇️
... and 17 more

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