Fix mypy typing errors in pytorch_lightning/strategies/single_tpu.py

[CyprienRicque]

What does this PR do?

Fixes mypy typing errors in pytorch_lightning/strategies/single_tpu.py in #13445.

Does your PR introduce any breaking changes? If yes, please list them.

This could be a breaking change in case I missed something/relative to older lightning versions.

in SingleTPUStrategy class, in function setup(self, trainer: "pl.Trainer"), the call to self.model.on_post_move_to_device() has been removed because the condition to trigger it cannot be True.
The condition is is_overridden("on_post_move_to_device", self.lightning_module). To be True, among other things, this condition requires the parent class to contain the on_post_move_to_device method. This on_post_move_to_device method is not present in any of the parent classes listed in is_overridden, therefore the condition cannot be True. (parents classes being LightningModule, LightningDataModule and Callback).

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