Add support for segmentation with different mask extensions

[ethanwharris]

What does this PR do?

Fixes #1129

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[codecov]

Codecov Report

Merging #1130 (f673ec0) into master (4997226) will decrease coverage by 0.00%.
The diff coverage is 100.00%.

@@            Coverage Diff             @@
##           master    #1130      +/-   ##
==========================================
- Coverage   88.84%   88.84%   -0.01%     
==========================================
  Files         286      286              
  Lines       12931    12927       -4     
==========================================
- Hits        11489    11485       -4     
  Misses       1442     1442              

Flag	Coverage Δ
unittests	88.84% <100.00%> (-0.01%)	⬇️

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

Impacted Files	Coverage Δ
flash/image/segmentation/data.py	100.00% <ø> (ø)
flash/image/segmentation/input.py	100.00% <100.00%> (ø)

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[AlexandreBrown]

Tested the changes and it works.
LGTM

[tchaton]

Very neat, tiny comments !