Use `DataPipe` not `IterDataPipe` as the type hints for `DataLoader2` related code

[ejguan]

🐛 Describe the bug

There are a couple of places that DataLoader2 uses IterDataPipe as the type hint (here, here, etc.)

We should add a type called DataPipe = Union[IterDataPipe, MapDataPipe] as the type hint, and use this new type hint for all functions.

Versions

main branch

[ejguan]

Let's not label it as the good first issue since the fix might need some work in the internal code base

[VitalyFedyunin]

Need to be very careful. As for example here https://github.com/pytorch/data/blob/12cfaf8899b1337981cd4edf9deef127f925f1bd/torchdata/dataloader2/reading_service.py#L17 is should be IterDataPipe as any MapDataPipe supposed to be wrapped to sampler before passing to DataLoader.

[ejguan]

Closing as it has been solved