Fix spawn plugins not deleting temp checkpoint

[awaelchli]

What does this PR do?

Fixes #10933
A temporary checkpoint gets created when Trainer(strategy="ddp_spawn") but never gets deleted.

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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 @awaelchli @ananthsub @ninginthecloud @justusschock @kaushikb11

[tchaton]

LGTM !