Fix trivial comparison in model checkpoint test

[ananthsub]

What does this PR do?

The model checkpoint test was keys across the same checkpoint dict instead of ckpt_last vs ckpt_last_epoch
This PR fixes that. All other changes here are formatting

Fixes # (issue)
Follows up on discussion in #2501

Before submitting

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• Did you make sure your PR does only one thing, instead of bundling different changes together? Otherwise, we ask you to create a separate PR for every change.
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[ananthsub]

this is the only logic change. all other changes are from auto-formatting

[awaelchli]

Thanks @ananthsub

[codecov]

Codecov Report

Merging #3464 into master will increase coverage by 5%.
The diff coverage is n/a.

@@           Coverage Diff           @@
##           master   #3464    +/-   ##
=======================================
+ Coverage      85%     90%    +5%     
=======================================
  Files         105     105            
  Lines        8157    8157            
=======================================
+ Hits         6943    7327   +384     
+ Misses       1214     830   -384