remove dataloader patching on the LightningModule

[awaelchli]

What does this PR do?

Fixes #6030
Closes #7522

Adds a new mechanism to fetch the dataloader only when it is needed. The previous way was to patch the LightningModule methods with a _PatchDataLoader. While this simplified the code internally for accessing/resetting dataloaders, it also causes problems with Mocking/Testing, debugging in general and silent bugs because the user is not aware that their methods get overridden.

The dataloader patching is nowhere documented. At this point we consider it a silent bug if the patched methods outlive the trainer and continue to be used in subsequent new trainer calls.
However, if someone relies on this weird logic (not sure what the use case would be), their code may break. I currently see no way to deprecate this or inform the user without resulting in false positive messages.

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[Borda]

lgtm 🐰