Mark trainer.callback_connector as protected

[daniellepintz]

What does this PR do?

Part of #9778

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[codecov]

Codecov Report

Merging #10121 (bd3ab12) into master (93266e2) will increase coverage by 0%.
The diff coverage is 100%.

❗ Current head bd3ab12 differs from pull request most recent head 7b10f61. Consider uploading reports for the commit 7b10f61 to get more accurate results

@@           Coverage Diff           @@
##           master   #10121   +/-   ##
=======================================
  Coverage      89%      89%           
=======================================
  Files         180      182    +2     
  Lines       16151    16149    -2     
=======================================
- Hits        14324    14323    -1     
+ Misses       1827     1826    -1