typing trainer.connectors.checkpoint connector

[jjenniferdai]

What does this PR do?

part of #5023, finish typing for trainer.connectors.checkpoint connector

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[tchaton]

LGTM !

[ananthsub]

fyi some overlap between this and #9434

[codecov]

Codecov Report

Merging #9419 (c8499dd) into master (3b34c89) will increase coverage by 0%.
The diff coverage is 100%.

@@          Coverage Diff           @@
##           master   #9419   +/-   ##
======================================
  Coverage      89%     89%           
======================================
  Files         180     180           
  Lines       15102   15106    +4     
======================================
+ Hits        13414   13418    +4     
  Misses       1688    1688