Add new PrivateUse1 DeviceType for non-public devices (#77208)
Summary: The new PrivateUse1 DeviceType is associated with the PrivateUse1 DispatchKey, which can be used for non-public devices without introducing a new device type. Note that the stringified name of the PrivateUse1 device is "privateuseone".
Test Plan: All CI should pass.
Differential Revision: D35859437
Pull Request resolved: https://github.com/pytorch/pytorch/pull/77208
Approved by: https://github.com/bdhirsh
