Definition of `IterDataPipe` in `pyi` file breaks inheritance path for static type checking

See comments in https://github.com/pytorch/data/pull/780

@pmeier
At list for the first Error, the proper typing should be:

def load(path: pathlib.Path) -> IterDataPipe[Tuple[str, BinaryIO]]:
    if path.is_dir():
        dp: IterDataPipe = FileLister(str(path), recursive=True)
    else:
        dp = IterableWrapper([str(path)])
    return FileOpener(dp, mode="rb")

However, even with the proper typing shown above, the Error is changed to Incompatible types in assignment (expression has type "FileListerIterDataPipe", variable has type "IterDataPipe[Any]"). And, it doesn't explain what causes the second Error.
So, I spent a few hours figuring out what is the root cause of the mypy Error. In the generated datapipe.pyi file, a new IterDataPipe class is defined, which overrides the original IterDataPipe from the inheritance graph for all other DataPipe.

All Errors are eliminated when I remove new IterDataPipe definition from datapipe.pyi and import IterDataPipe directly from torch.utils.data.datapipe. And, the reason we define new IterDataPipe in pyi file is attaching all functional APIs to it. We need to do it in a different way by keeping the original IterDataPipe and extending the class with all functional APIs.

cc: @NivekT for python interface file

For this PR, I will revert it because our typing system needs to be fixed generally.

Originally posted by @ejguan in https://github.com/pytorch/data/issues/780#issuecomment-1252595095

Start with the generated datapipe.pyi file and the IterDataPipe definition imported from torch.utils.data.datapipe, then review the comments in pull request 780 and reproduce the reported mypy errors. The fix is complete when the original inheritance path is preserved, functional APIs remain available, and the shown typing errors are eliminated.