Reset val_dataloader in tuner/batch_size_scaling

[rohitgr7]

What does this PR do?

Fixes #9832

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[codecov]

Codecov Report

Merging #9857 (801e221) into master (4decbc0) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #9857   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         178     178           
  Lines       15618   15619    +1     
======================================
+ Hits        14475   14476    +1     
  Misses       1143    1143           

[tchaton]

LGTM !