[DataPipe] Add generated docstring to functional form DataPipe (#100503)
This PR modified the generation process of `datapipe.pyi` to include the doc strings for each DataPipe in functional form.
The new generated `.pyi` file will look like [this](https://gist.github.com/NivekT/95095f14da85a837a0727a19a5ba367c). I have confirmed the doc string will be visible in PyCharm.
You can copy this [file](https://gist.github.com/NivekT/95095f14da85a837a0727a19a5ba367c) and overwrite your local `datapipe.pyi` to validate this change as well.
Note: We need to create a similar change in TorchData to allow DataPipes in that library to show the doc strings as well.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/100503
Approved by: https://github.com/ejguan
