[BugFix] Add on_epoch_end hook after on_test/validation_epoch_end hook

[tchaton]

What does this PR do?

This PR add missing on_epoch_end hook after validation / test epoch.

Fixes #5917

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[codecov]

Codecov Report

Merging #5986 (9f9ad34) into master (f655f97) will increase coverage by 2%.
The diff coverage is 98%.

@@           Coverage Diff           @@
##           master   #5986    +/-   ##
=======================================
+ Coverage      91%     93%    +2%     
=======================================
  Files         160     160            
  Lines       11313   11341    +28     
=======================================
+ Hits        10272   10523   +251     
+ Misses       1041     818   -223     

[awaelchli]

it looks like there is on_test_epoch_end but no on_train_epoch_end
edit: no it's there 😄