Fixed a bug where validation metrics could be aggregated together with test metrics

[ethanwharris]

What does this PR do?

Fixes #843

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[codecov]

Codecov Report

Merging #900 (9c3257a) into master (194c8d9) will increase coverage by 0.07%.
The diff coverage is 100.00%.

❗ Current head 9c3257a differs from pull request most recent head 136a476. Consider uploading reports for the commit 136a476 to get more accurate results

@@            Coverage Diff             @@
##           master     #900      +/-   ##
==========================================
+ Coverage   89.01%   89.08%   +0.07%     
==========================================
  Files         230      230              
  Lines       12719    12709      -10     
==========================================
  Hits        11322    11322              
+ Misses       1397     1387      -10     

Flag	Coverage Δ
unittests	89.08% <100.00%> (+0.07%)	⬆️

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

Impacted Files	Coverage Δ
flash/core/model.py	88.20% <100.00%> (+0.02%)	⬆️
...ash/image/classification/integrations/baal/data.py	40.20% <0.00%> (-1.29%)	⬇️
...ash/image/classification/integrations/baal/loop.py	35.55% <0.00%> (+3.82%)	⬆️

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