Move _dummy_type to _utils.py (#40177)
Summary:
Use it from both __init__ and streams to define dummy types when CUDA is missing
Fix accidental reference of global `storage_name` from `_dummy_type`
Add type annotations
Pull Request resolved: https://github.com/pytorch/pytorch/pull/40177
Differential Revision: D22106922
Pulled By: malfet
fbshipit-source-id: 52fbfd91d70a78eb14d7ffda109c02ad1231497e
