Fix serve sanity checking

[ethanwharris]

What does this PR do?

Fixes #492

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[tchaton]

LGTM !

[codecov]

Codecov Report

Merging #493 (fba0aea) into master (da5b5a8) will decrease coverage by 0.03%.
The diff coverage is 50.00%.

@@            Coverage Diff             @@
##           master     #493      +/-   ##
==========================================
- Coverage   91.46%   91.42%   -0.04%     
==========================================
  Files         112      112              
  Lines        7098     7103       +5     
==========================================
+ Hits         6492     6494       +2     
- Misses        606      609       +3     

Flag	Coverage Δ
unittests	91.42% <50.00%> (-0.04%)	⬇️

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

Impacted Files	Coverage Δ
flash/core/trainer.py	84.61% <50.00%> (-2.60%)	⬇️

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