[ROCm] float16.h should use __HIP__ not USE_ROCM (#13684)
The float16.h header is shared between the CPU and ROCm EPs. The
USE_ROCM macro is defined universally, but for the float16.h header we
only wish to detect the hip-clang compiler. Otherwise, the CPU EP fails
to build because of -Werror -Wuninitialized caused by the USE_ROCM code
additions, and the CPU EP should be using a different code path.
