Set and get default dtype (#13748)

Set and get default dtype (#13748)

Summary:
Replaces the `DefaultTensorOptions` with just a global default dtype that you can set and get like in Python.

Also, calls `set_default_dtype` in the implementation of `torch.set_default_dtype`. Right now these two default values are separate but will always be the same. Should we just bind `set_default_dtype`  into Python? I think that might be good to do in a separate PR though.

ezyang gchanan

Also CC colesbury who wanted to do this for ATen for a while? What do you think about it?
Pull Request resolved: https://github.com/pytorch/pytorch/pull/13748

Differential Revision: D13340207

Pulled By: goldsborough

fbshipit-source-id: 2689b09eb137fabb3a92d1ad1635782bee9398e8