Migrate THCState to ATen (#66480)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66480
This guts `THCState` to simply be an empty struct, as well as:
- moving `THCState_getPeerToPeerAccess` and its cache into `ATen`.
- cleaning up dead code in `THCGeneral.cpp`
- moving `THCudaInit` and `THCMagma_init` into `CUDAHooks::initCUDA`
Test Plan: Imported from OSS
Reviewed By: H-Huang
Differential Revision: D31577488
Pulled By: ngimel
fbshipit-source-id: 90604f30854fe766675baa3863707ac09995bc9e
