Fix pickling torch.float32 (#18045)
Summary:
Attempt fix for #14057 . This PR fixes the example script in the issue.
The old behavior is a bit confusing here. What happened to pickling is python2 failed to recognize `torch.float32` is in module `torch`, thus it's looking for `torch.float32` in module `__main__`. Python3 is smart enough to handle it.
According to the doc [here](https://docs.python.org/2/library/pickle.html#object.__reduce__), it seems `__reduce__` should return `float32` instead of the old name `torch.float32`. In this way python2 is able to find `float32` in `torch` module.
> If a string is returned, it names a global variable whose contents are pickled as normal. The string returned by __reduce__() should be the object’s local name relative to its module
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18045
Differential Revision: D14990638
Pulled By: ailzhang
fbshipit-source-id: 816b97d63a934a5dda1a910312ad69f120b0b4de
