Remove model.trainer call inside of dataloading mixin

[ananthsub]

What does this PR do?

Part of better state management: this is a mixin on the trainer. there's no reason for us to use the model's trainer attribute to call this hook. it's a code smell we will redesign as part of #7315

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[codecov]

Codecov Report

Merging #7317 (2115bd3) into master (393b252) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #7317    +/-   ##
=======================================
- Coverage      91%     87%    -4%     
=======================================
  Files         200     200            
  Lines       12875   12875            
=======================================
- Hits        11765   11220   -545     
- Misses       1110    1655   +545