Fix mypy errors attributed to `pytorch_lightning.core.datamodule`

[jxtngx]

What does this PR do?

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

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[jxtngx]

blocked by core.saving of #13445

[jxtngx]

I had originally marked this blocked by another contributor's anticipated PR; however, that contributor has not opened a PR after 2 weeks https://github.com/Lightning-AI/lightning/issues/13445#issuecomment-1185436344

[codecov[bot]]

Codecov Report

Merging #13693 (402d025) into master (0ca3b5a) will decrease coverage by 10%. The diff coverage is 96%.

:exclamation: Current head 402d025 differs from pull request most recent head e7cef8a. Consider uploading reports for the commit e7cef8a to get more accurate results

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[jxtngx]

@otaj @carmocca @Borda @rohitgr7

The new, suggested class methods in LightningDataModule are to reconcile errors raised by mypy; however, these errors originate from pl.utilities.argparse.

While troubleshooting the mypy error:

src/pytorch_lightning/utilities/argparse.py:65: error: Item "LightningDataModule" of "Union[ParseArgparserDataType, LightningDataModule]" has no attribute "parse_argparser"  [union-attr]

It was found that in pl.utilities.argparse, there appears to be an unused base class (ParseArgparserDataType) that was perhaps intended to act as a mixin/utility interface for LightningDataModule and Trainer. This base class originated in https://github.com/Lightning-AI/lightning/pull/8124, as pointed out by Ota in a Slack conversation.

Should I integrate ParseArgparserDataType in this PR – refactoring both LightningDataModule and Trainer appropriately to inherit from the base class; or should I open a new PR to handle this task?

If this PR should not integrate ParseArgparserDataType, then it is ready for review; otherwise, it should be converted to a draft.

all level 0 functions in pl.utilities.argparse appear as if each should be refactored as either a class or static method of ParseArgparserDataType

[jxtngx]

@rohitgr7 what was the intended effect of setting strict to None? strict needs to be a bool, so I am wondering if this needs to be False or left as none and ignore the mypy error https://github.com/Lightning-AI/lightning/blob/d29a552b3c701ebc14d608347c1dbf55c3dfaa6a/src/pytorch_lightning/core/datamodule.py#L245

_load_from_checkpoint was updated in the PR for core.saving yesterday.

@awaelchli this will likely affect your update to core.saving as well

[rohitgr7]

@JustinGoheen added a commit to resolve the issue

[jxtngx]

apologies, I'm no longer able to keep this PR up to date.