segmentation_models.pytorch integration

[aniketmaurya]

What does this PR do?

Fixes #515 #516

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[codecov]

Codecov Report

Merging #562 (7121dfb) into master (c318e4a) will decrease coverage by 1.41%.
The diff coverage is 96.55%.

@@            Coverage Diff             @@
##           master     #562      +/-   ##
==========================================
- Coverage   91.27%   89.86%   -1.42%     
==========================================
  Files         117      117              
  Lines        7234     7191      -43     
==========================================
- Hits         6603     6462     -141     
- Misses        631      729      +98     

Flag	Coverage Δ
unittests	89.86% <96.55%> (-1.42%)	⬇️

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

Impacted Files	Coverage Δ
flash/image/segmentation/data.py	81.33% <ø> (ø)
flash/image/segmentation/heads.py	94.73% <93.33%> (+16.77%)	⬆️
flash/core/utilities/imports.py	89.65% <100.00%> (+0.12%)	⬆️
flash/image/segmentation/backbones.py	100.00% <100.00%> (ø)
flash/image/segmentation/model.py	89.70% <100.00%> (+1.64%)	⬆️
flash/text/seq2seq/core/metrics.py	74.07% <0.00%> (-23.15%)	⬇️
flash/text/classification/model.py	70.45% <0.00%> (-22.73%)	⬇️
flash/text/classification/data.py	66.66% <0.00%> (-22.43%)	⬇️
flash/text/seq2seq/core/data.py	72.00% <0.00%> (-20.00%)	⬇️
flash/text/seq2seq/core/model.py	66.66% <0.00%> (-8.70%)	⬇️
... and 4 more

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[tchaton]

LGTM !

[ethanwharris]

Awesome! LGTM 😃