[bugfix] Resolve metrics not being properly resetted on validation epoch end

[tchaton]

What does this PR do?

Fixes #9683

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[codecov]

Codecov Report

Merging #9717 (404cd80) into master (ab06987) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #9717    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         178     178            
  Lines       15363   15366     +3     
=======================================
- Hits        14228   13617   -611     
- Misses       1135    1749   +614