Fix drop last for predicting and testing

[ethanwharris]

What does this PR do?

Fixes # (issue)

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[codecov]

Codecov Report

Merging #671 (65791b8) into master (9b86a0f) will not change coverage.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master     #671   +/-   ##
=======================================
  Coverage   90.01%   90.01%           
=======================================
  Files         185      185           
  Lines        9664     9664           
=======================================
  Hits         8699     8699           
  Misses        965      965           

Flag	Coverage Δ
unittests	90.01% <ø> (ø)

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

Impacted Files	Coverage Δ
flash/core/model.py	86.28% <ø> (ø)

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