Add Half conversion of bit cast for SYCL kernel (#64340)

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