Use checkpoint_connector.hpc_save in SLURMConnector

[EspenHa]

What does this PR do?

Fixes #4208. Simply change SLURMConnector to use trainer.checkpoint_connector.hpc_save instead of trainer.hpc_save.

Note that I have not tested this as I don't have access to a SLURM cluster at the moment.

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Did you have fun?

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[codecov]

Codecov Report

Merging #4217 into master will not change coverage.
The diff coverage is n/a.

@@          Coverage Diff           @@
##           master   #4217   +/-   ##
======================================
  Coverage      93%     93%           
======================================
  Files         103     103           
  Lines        7848    7848           
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  Hits         7281    7281           
  Misses        567     567           

[andreamad8]

I just tested, it works 👍🏻