[BE] Add lazy_import

🚀 Feature

Currently we used import in each DataPipe class to make lazy importing happens like https://github.com/pytorch/data/blob/4802a350d1bf954d0785e0f22fd1fcde2deded76/torchdata/datapipes/iter/load/iopath.py#L21-L29

As more potential libraries used in TorchData to support different functionalities, we could add a methods to support lazy import module to global namespace. Then, we don’t need to duplicate the import inside each class used the same third-party module.

Features needed:

• Error message generation
• Support from ... import ... as ...
• Support submodule lazy import import xxx.yyy

[erip]

So you want to stick with the current, but wrap up the try/except logic?

class XXXDataPipe:
    def __init__(self, ...):
        self._abc = lazy_import("abc")

    def __iter__(self):
        self._abc

[ejguan]

In other words, attribute access on not-installed modules should never happen?

Yeah, you are right. The Error should be raised at the first place. Otherwise, it should be a bug.