[bugfix] Resolve memory leak for evaluation

[tchaton]

What does this PR do?

This PR delete self.trainer.evaluation_loop.outputs on validation_epoch_end call.

Fixes #5735

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[codecov]

Codecov Report

Merging #6326 (e2d179d) into master (49c579f) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #6326   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         160     160           
  Lines       11415   11416    +1     
======================================
+ Hits        10644   10645    +1     
  Misses        771     771           

[carmocca]

Please keep the logging_ directory name as that's done on purpose to not collide with the stdlib logging