Improve support for custom DataLoaders when instantiated in *_dataloader hook

[otaj]

What does this PR do?

Fixes #12917
Improve support for custom DataLoaders when instantiated in *_dataloader hook. If the DataLoader is a valid DataLoader, there is now a lower chance that things we will break the DataLoader when injecting custom arguments.

Also, if things break due to our injecting, we throw better error messages.

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

DataLoader instantiated inside a hook will now not instantiate attributes for you automatically. Before this PR, this code would work:

class MyDataLoader(DataLoader):
    def __init__(dataset, my_arg, **kwargs):
        # we explicitly do not set my_arg, as PL sets it for us automatically
        super().__init__(dataset, **kwargs)

class MyModule(pl.LightningModule):
    ...
    def train_dataloader(self):
        loader = MyDataLoader(range(10), 10, batch_size=2)
        assert loader.my_arg == 10
        return loader

After this PR, this code would no longer work. This was an internal behavior.

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cc @Borda @justusschock @awaelchli @ninginthecloud @rohitgr7 @otaj

[otaj]

I think I replied to all of your comments @carmocca. I applied most of your suggestions, would you mind taking another look? Thanks!