Remove PythonOp non-CPU path and PytorchOp (#15417)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15417
Right now the way we test whether Blob contains a CPU tensor is broken in ```PythonOpBase``` is broken, which means non-CPU path might never be taken.
Searching through the codebase, non-gpu path is used in PythonDLPack, and it is used in PytorchOp which is unused. So we'll remove non-gpu path in this diff.
Reviewed By: dzhulgakov
Differential Revision: D13495011
fbshipit-source-id: 9fe9537f05026d2a2cf7051efa81d184de722710
