What does this PR do?

Fixes #13502 (main RFC)
Fixes #8593
Fixes #13500

The PR looks big, but most complexity turns out to be in the deprecation. The rest of the changes are very thinly spread across multiple files, but are not complex by themselves.

Before, you would do

lightning_module = unwrap_lightning_module(self.model)

Now, directly access it:

lightning_module = self.lightning_module

Terminology:
self.model: refers to a LightningModule that is potentially wrapped
self.lightning_module: is always the pure LightningModule as it gets passed to Trainer.fit by the user. This attribute is read-access only.

TODOs:

• backward compatibility in wrappers/deprecation

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

No.

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I made sure I had fun coding 🙃

cc @Borda @justusschock @awaelchli @rohitgr7 @tchaton @akihironitta