Fix mypy errors attributed to pytorch_lightning.core.saving

[jxtngx]

What does this PR do?

Fix mypy errors attributed to pytorch_lightning.core.saving for issue #13445

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[jxtngx]

@awaelchli @carmocca @Borda @otaj I made a few changes to standardize annotations for map_location across all files that use load_checkpoint. The new annotation uses Carlos' recent change in pl.utilities.cloud_io and adds Callable into the annotation based on doc strings for checkpoint_plugin.CheckpointIO.load_checkpoint describing map_location. The new annotation is made available in types as MAP_LOCATION.

new changes have been added

[awaelchli]

IMO, the previous version was in a much better state and ready to merge. I think we should leave refactors and re-definitions in unrelated files for separate PRs.

[carmocca]

@Borda I believe this was incorrectly milestoned

[Borda]

@Borda I believe this was incorrectly milestoned

well yes, I think it can be any release, just was thinking that all this typing included would simplify eventual future cherry-picking as it lowers the diff between master and releases branch

[carmocca]

There's some risk involved in typing PRs (example) so we want to minimize it for bug-fix releases.