DataPipeline Refactor V2: Simplify Input Transform

[tchaton]

What does this PR do?

Fixes #924

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[codecov]

Codecov Report

Merging #1010 (9c3d408) into master (43cfb3d) will decrease coverage by 0.04%.
The diff coverage is 92.30%.

❗ Current head 9c3d408 differs from pull request most recent head 1eeae27. Consider uploading reports for the commit 1eeae27 to get more accurate results

@@            Coverage Diff             @@
##           master    #1010      +/-   ##
==========================================
- Coverage   88.29%   88.25%   -0.05%     
==========================================
  Files         254      254              
  Lines       13506    13422      -84     
==========================================
- Hits        11925    11845      -80     
+ Misses       1581     1577       -4     

Flag	Coverage Δ
unittests	88.25% <92.30%> (-0.05%)	⬇️

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

Impacted Files	Coverage Δ
flash/audio/classification/transforms.py	86.36% <0.00%> (-0.60%)	⬇️
flash/core/data/data_module.py	95.42% <ø> (ø)
flash/core/data/io/input.py	69.70% <ø> (ø)
flash/core/integrations/icevision/transforms.py	90.27% <ø> (ø)
flash/image/face_detection/data.py	51.56% <ø> (ø)
flash/image/segmentation/transforms.py	100.00% <ø> (ø)
flash/text/classification/data.py	96.90% <ø> (ø)
flash/text/question_answering/data.py	87.02% <ø> (ø)
flash/image/classification/transforms.py	93.75% <60.00%> (ø)
flash/core/data/base_viz.py	100.00% <100.00%> (ø)
... and 15 more

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[ethanwharris]

LGTM, minor comment