Mark trainer.config_validator as protected

[ananthsub]

What does this PR do?

Part of #9778

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[codecov]

Codecov Report

Merging #9779 (4dbd17d) into master (5206e52) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #9779    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         178     178            
  Lines       15620   15620            
=======================================
- Hits        14477   13868   -609     
- Misses       1143    1752   +609     

[tchaton]

LGTM !