Fix double iteration bug when resumed from a checkpoint.

[sudiptob2]

What does this PR do?

This PR fixes the double iter() bug discussed in #19427

Fixes #19427

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📚 Documentation preview 📚: https://pytorch-lightning--20775.org.readthedocs.build/en/20775/

[sudiptob2]

@Borda Thanks for the review! Let me know if there's anything you'd like me to change. Otherwise, can we go ahead and merge this?

[sudiptob2]

Hey, @Borda just wondering if anything is blocking this PR from merging?

[Borda]

Looking at how the _loaded_from_state_dict is used, and there is no direct wrap for loading, meaning set it as true/false when loading starts, and switch back when it ends so for that we my need to add another/new attribute

[sudiptob2]

Hi! Thanks for taking a look at it. I am not sure why we need that. _loaded_from_state_dict is already made True and False when necessary so I am using _loaded_from_state_dict to detect checkpoint resumption.
I might have missed something, could you please suggest what changes we need here?

pytorch-lightning/src/lightning/pytorch/loops/loop.py

Line 89 in 1e88899

self._loaded_from_state_dict = True

pytorch-lightning/src/lightning/pytorch/loops/loop.py

Lines 102 to 105 in 1e88899

	def on_iteration_done(self) -> None:
	self._restarting = False
	self._loaded_from_state_dict = False
	self.reset_restart_stage()