Mark trainer.accelerator_connector as protected

[daniellepintz]

What does this PR do?

Part of #9778

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[codecov]

Codecov Report

Merging #10032 (cca90a0) into master (76081fb) will decrease coverage by 4%.
The diff coverage is 75%.

❗ Current head cca90a0 differs from pull request most recent head e95b17e. Consider uploading reports for the commit e95b17e to get more accurate results

@@           Coverage Diff            @@
##           master   #10032    +/-   ##
========================================
- Coverage      93%      89%    -4%     
========================================
  Files         182      182            
  Lines       16157    16111    -46     
========================================
- Hits        14947    14273   -674     
- Misses       1210     1838   +628     

[tchaton]

LGTM !