Remove deviceTypeToBackend, which is underspecified. (#18135)

Summary:
There are multiple backends for a device type, so we just kill this function.
Also, kill an getNonVariableType instance which was also underspecified.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18135

Differential Revision: D14507474

Pulled By: gchanan

fbshipit-source-id: fc791a76d4b851b23d09a070725f3838621eb13d