Pytorch-lightning: Warn when you forget to call super().__init__() in datamodule subclass

[nateraw]

🚀 Feature

If you forget to call super().__init__() in datamodule subclass, you end up getting confusing errors down the line. We should let the user know they've forgotten to do so and give them a warning.

Motivation

Pitch

Alternatives

Additional context

[rbracco]

Exact error message is: AttributeError: 'YourDataModule' object has no attribute '_has_prepared_data'. Adding this to make the error searchable here.

[stale[bot]]

This issue has been automatically marked as stale because it hasn't had any recent activity. This issue will be closed in 7 days if no further activity occurs. Thank you for your contributions, Pytorch Lightning Team!