Add Accelerator device and shell hooks (#119329)
This adds a concept of Accelerator that points to one of our devices. See DeviceAccelerator.h in this PR for details https://github.com/pytorch/pytorch/pull/119329/files#diff-83cc748bed5df1a453c272cc5ecc7e572d4eb694c5125384d8fbd17a0b5f50c8
It also adds scaffolding for shared C++ API to allow generic feature implementation. This PR in particular updates the autograd engine to use this generic API.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/119329
Approved by: https://github.com/ezyang
