Add batch transform integ tests

[laurenyu]

Description of changes:
This change introduces a couple batch transform integ tests. Since there is the general implementation of transformer() with EstimatorBase as well as one specific to Framework, I added one test for each of the two implementations. One of the tests also covers Transformer.attach().

Merge Checklist

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• I have read the CONTRIBUTING doc
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[codecov-io]

Codecov Report

Merging #354 into master will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #354   +/-   ##
=======================================
  Coverage   92.93%   92.93%           
=======================================
  Files          51       51           
  Lines        3552     3552           
=======================================
  Hits         3301     3301           
  Misses        251      251

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