Add PreprocessTransform

[tchaton]

What does this PR do?

This PR is part of an internal refactor of the DataPipeline.

Fixes # (issue)

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[codecov]

Codecov Report

Merging #852 (64e376d) into master (0449611) will increase coverage by 0.03%.
The diff coverage is 92.02%.

@@            Coverage Diff             @@
##           master     #852      +/-   ##
==========================================
+ Coverage   89.04%   89.08%   +0.03%     
==========================================
  Files         225      226       +1     
  Lines       12186    12348     +162     
==========================================
+ Hits        10851    11000     +149     
- Misses       1335     1348      +13     

Flag	Coverage Δ
unittests	89.08% <92.02%> (+0.03%)	⬆️

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

Impacted Files	Coverage Δ
flash/core/data/process.py	87.70% <ø> (-0.06%)	⬇️
flash/core/data/preprocess_transform.py	92.02% <92.02%> (ø)

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