Simplify checkpoint connector loading after Checkpoint IO plugin's introduction

[ananthsub]

What does this PR do?

• Moves the validating logic of checking for the path's existence from the checkpoint connector to the torch checkpoint IO plugin (where the fsspec logic lives)
• Moves the CUDA cache clearing from the checkpoint connector to the training type plugin
• Renames load_checkpoint_file in the training type to load_checkpoint for consistency with the checkpoint IO's API

Does your PR introduce any breaking changes? If yes, please list them.

This is a breaking change for any custom plugin that extended load_checkpoint_file as this has been renamed to load_checkpoint with this PR

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[codecov]

Codecov Report

Merging #9045 (dbb65b9) into master (8a93173) will decrease coverage by 4%.
The diff coverage is 91%.

@@           Coverage Diff           @@
##           master   #9045    +/-   ##
=======================================
- Coverage      93%     89%    -4%     
=======================================
  Files         175     175            
  Lines       14379   14457    +78     
=======================================
- Hits        13322   12806   -516     
- Misses       1057    1651   +594     

[tchaton]

Neat !