Fix & Implement xdoctest

[ejguan]

📚 The doc issue

There is a PR https://github.com/pytorch/pytorch/pull/82797 landed into PyTorch core, which adds the functionality to validate if the example in comment is runnable.

However, in the example of PyTorch Core, we normally refer torchdata in all examples for the sake of unification of importing path rather than directly importing DataPipes from pytorch core. This would cause xdoctest always failing. TBH, I don't know how to solve this problem without changing it back to import torch.data.utils.....

But, for torchdata project, we can do the similar work as a BE project to enable all doc test over the examples to prevent any failing test in our documentation.

Suggest a potential alternative/fix

No response