Added FPGA DispatchKey, DeviceType, Backend (#38938)
Summary:
ezyang,
I have added the changes to DispatchKey, DeviceType, Backend to support the out-of-tree FPGA.
cc. tataetae
Pull Request resolved: https://github.com/pytorch/pytorch/pull/38938
Differential Revision: D21748955
Pulled By: ezyang
fbshipit-source-id: fe76d9730818205961430d2a0e00727b5c547b32
