Support NP_EXTENSIONS for SemanticSegmentationFilesInput

[krshrimali]

What does this PR do?

As per the title. Fixes #1308.

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[codecov]

Codecov Report

Merging #1369 (496de60) into master (fd8cc7f) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1369   +/-   ##
=======================================
  Coverage   92.93%   92.93%           
=======================================
  Files         285      285           
  Lines       12778    12778           
=======================================
  Hits        11875    11875           
  Misses        903      903           

Flag	Coverage Δ
unittests	92.93% <100.00%> (ø)

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Impacted Files	Coverage Δ
flash/image/segmentation/data.py	100.00% <ø> (ø)
flash/core/data/utilities/loading.py	97.10% <100.00%> (ø)
flash/image/segmentation/input.py	100.00% <100.00%> (ø)

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