Fix type hint for filepath {save/load/remove}_checkpoint

[kaushikb11]

What does this PR do?

Fixes type hint for filepath {save/load/remove}_checkpoint

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[codecov]

Codecov Report

Merging #9434 (86477b0) into master (e0f2e04) will decrease coverage by 4%.
The diff coverage is 100%.

@@           Coverage Diff           @@
##           master   #9434    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         179     179            
  Lines       14922   14924     +2     
=======================================
- Hits        13861   13251   -610     
- Misses       1061    1673   +612