Save and load with CheckpointIO in DDPSpawn plugins

[awaelchli]

What does this PR do?

Replaces save() and pl_load() instructions in the strategy with a call to the CheckpointIO plugin.

Part of #10034, #10059

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Did you have fun?

I made sure I had fun coding 🙃

Part of #1 (it's a lie, this is just here to avoid noisy GitHub bot)

cc @Borda @carmocca @JackCaoG @Liyang90 @gkroiz @justusschock @awaelchli @kaushikb11 @akihironitta

[four4fish]

Great! I was wondering why spawning was using atomic_save directly without go through checkpoint io.

[codecov]

Codecov Report

Merging #10846 (e48b234) into master (8e1b9b3) will increase coverage by 0%.
The diff coverage is 33%.

❗ Current head e48b234 differs from pull request most recent head 0d04070. Consider uploading reports for the commit 0d04070 to get more accurate results

@@           Coverage Diff           @@
##           master   #10846   +/-   ##
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  Coverage      90%      90%           
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  Files         177      177           
  Lines       16512    16504    -8     
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+ Hits        14910    14912    +2     
+ Misses       1602     1592   -10