Split up transform into its own module.

[jaheba]

*Issue: #481

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[codecov-io]

Codecov Report

Merging #499 into master will increase coverage by 0.11%.
The diff coverage is 91.01%.

@@            Coverage Diff             @@
##           master     #499      +/-   ##
==========================================
+ Coverage   83.75%   83.87%   +0.11%     
==========================================
  Files         159      166       +7     
  Lines        9469     9507      +38     
==========================================
+ Hits         7931     7974      +43     
+ Misses       1538     1533       -5

Impacted Files	Coverage Δ
src/gluonts/model/wavenet/_estimator.py	95.95% <100%> (ø)	⬆️
src/gluonts/transform/dataset.py	100% <100%> (ø)
src/gluonts/shell/train.py	90.47% <100%> (+0.23%)	⬆️
src/gluonts/model/seq2seq/_transform.py	96.42% <100%> (+0.06%)	⬆️
src/gluonts/transform/_base.py	86.66% <86.66%> (ø)
src/gluonts/transform/convert.py	86.84% <86.84%> (ø)
src/gluonts/transform/feature.py	89.77% <89.77%> (ø)
src/gluonts/transform/__init__.py	90.9% <90.9%> (ø)
src/gluonts/transform/sampler.py	91.66% <91.66%> (ø)
src/gluonts/transform/field.py	94.23% <94.23%> (ø)
... and 9 more