* Split set_default_tensor_type(dtype) into set_default_dtype(dtype).
* Fix flake8.
The difference between this one and set_default_tensor_type is that it only sets scalar type what determines the type + device of a tensor returned from a factory function with defaults is the default tensor type + the current device (if the default tensor type is cuda). This just changes the scalar type of the default tensor type.
We do eventually want to deprecate set_default_tensor_type; it is not clear how to do that in a sensible and backwards compatible way.