Change serve sanity check default to False

[ethanwharris]

What does this PR do?

Serve currently doesn't work from a notebook so the sanity check will fail. This changes the default to false so that the sanity check won't run unless requested

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[codecov]

Codecov Report

Merging #447 (05b8835) into master (882e204) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #447   +/-   ##
=======================================
  Coverage   91.66%   91.66%           
=======================================
  Files         112      112           
  Lines        7065     7065           
=======================================
  Hits         6476     6476           
  Misses        589      589           

Flag	Coverage Δ
unittests	91.66% <100.00%> (ø)

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

Impacted Files	Coverage Δ
flash/core/trainer.py	87.20% <100.00%> (ø)

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