[feat] Update Segmentation Task in preparation to adding resizing

[tchaton]

What does this PR do?

This PR updates the Segmentation Task and simplify the internals.

Before submitting

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• Did you write any new necessary tests? [not needed for typos/docs]
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PR review

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Did you have fun?

Make sure you had fun coding 🙃

[ethanwharris]

This really cool, you found another use for state! 😃

[codecov]

Codecov Report

Merging #287 (bdec71f) into master (e36a250) will increase coverage by 4.11%.
The diff coverage is 88.00%.

@@            Coverage Diff             @@
##           master     #287      +/-   ##
==========================================
+ Coverage   84.28%   88.39%   +4.11%     
==========================================
  Files          74       74              
  Lines        3577     3612      +35     
==========================================
+ Hits         3015     3193     +178     
+ Misses        562      419     -143     

Flag	Coverage Δ
unittests	88.39% <88.00%> (+4.11%)	⬆️

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

Impacted Files	Coverage Δ
flash/data/data_pipeline.py	92.75% <50.00%> (+0.39%)	⬆️
flash/vision/segmentation/serialization.py	82.22% <60.00%> (+13.33%)	⬆️
flash/data/splits.py	92.85% <88.88%> (-1.88%)	⬇️
flash/vision/segmentation/data.py	88.74% <93.10%> (+0.10%)	⬆️
flash/__init__.py	100.00% <100.00%> (ø)
flash/data/data_source.py	96.85% <100.00%> (+2.54%)	⬆️
flash/data/properties.py	83.11% <0.00%> (+1.29%)	⬆️
flash/data/data_module.py	92.34% <0.00%> (+1.43%)	⬆️
... and 12 more

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