Added deviceCount() virtual method to DeviceGuardImplInterface (#15574)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/15574
Added deviceCount() virtual method to DeviceGuardImplInterface, also added correspondent implementation for CPUGuardImpl, CUDAGuardImpl, FakeGuardImpl, VirtualGuardImpl, HIPGuardImplMasqueradingAsCUDA
Reviewed By: soumith
Differential Revision: D13554609
fbshipit-source-id: 913bf2aad44a0a356efe54505ee4abaf6c4622db
