Add Half conversion of bit cast for SYCL kernel (#64340)
Summary:
## Motivation
Enhance the performance of Half/float conversion in SYCL kernels.
## Solution
Add the native SYCL half type to help convert the half from/to float in the kernel code.
## Additional Context
`__SYCL_DEVICE_ONLY__` is a MACRO only valid when compiling the kernel code for SYCL backend.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/64340
Reviewed By: gchanan
Differential Revision: D30720823
Pulled By: ezyang
fbshipit-source-id: e7e770d02df5b2d45da61d2fed3ba59383b3dc3a
