Profile Trainer.save_checkpoint

[EricCousineau-TRI]

What does this PR do?

Adds Profiler tracepoints to Trainer.save_checkpoint.

Did not file an issue (seems simple), but can do so if discussion is needed.

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  Minor change.

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📚 Documentation preview 📚: https://pytorch-lightning--20405.org.readthedocs.build/en/20405/

[lantiga]

LGTM, thank you!

[Borda]

lets add simple test too?