Refactor Segmentation models

[ekagra-ranjan]

This PR uses a protected method for loading and initializing the segmentation models. Relevant #875

[codecov-io]

Codecov Report

Merging #1009 into master will increase coverage by 0.34%.
The diff coverage is 87.5%.

@@            Coverage Diff             @@
##           master    #1009      +/-   ##
==========================================
+ Coverage   62.63%   62.98%   +0.34%     
==========================================
  Files          65       65              
  Lines        5101     5073      -28     
  Branches      765      756       -9     
==========================================
  Hits         3195     3195              
+ Misses       1683     1661      -22     
+ Partials      223      217       -6

Impacted Files	Coverage Δ
torchvision/models/segmentation/segmentation.py	67.44% <87.5%> (+25.18%)	⬆️
torchvision/transforms/transforms.py	81.25% <0%> (-0.65%)	⬇️
torchvision/transforms/functional.py	71.12% <0%> (+1.21%)	⬆️

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[fmassa]

Looks much better, thanks a lot!