Fix metric computation

[ethanwharris]

What does this PR do?

Fixes # (issue)

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[codecov]

Codecov Report

Merging #559 (e0d66c6) into master (bf6da22) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master     #559   +/-   ##
=======================================
  Coverage   91.31%   91.31%           
=======================================
  Files         112      112           
  Lines        7127     7128    +1     
=======================================
+ Hits         6508     6509    +1     
  Misses        619      619           

Flag	Coverage Δ
unittests	91.31% <100.00%> (+<0.01%)	⬆️

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

Impacted Files	Coverage Δ
flash/core/model.py	85.90% <100.00%> (+0.03%)	⬆️
flash/text/classification/model.py	93.18% <100.00%> (ø)
flash/video/classification/model.py	86.41% <100.00%> (ø)

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[tchaton]

LGTM !