Fix compile error of OptionalCUDAGuard and device_of

[reyoung]

There are some compile errors in the main branch, like

/app/python/csrc_aot/single_prefill.cu(59): error: namespace "at::cuda" has no member "OptionalCUDAGuard"
    const at::cuda::OptionalCUDAGuard device_guard(device_of(device));
                    ^

/app/python/csrc_aot/single_prefill.cu(59): error: identifier "device_of" is undefined
    const at::cuda::OptionalCUDAGuard device_guard(device_of(device));

cc @yzh119

[reyoung]

1. device_of is in a private header of torch.

2. It seems that OptionalCUDAGuard is not necessary since the device is not optional.

3. Adding include file for CUDAGuard

[yzh119]

LGTM, thank you @reyoung !
btw, I'm moving most of the code before kernel launch (including device guard) from C++ to python, but let's merge this first.

[zhyncs]

Thanks for this fix! ref #611
I’ll help add build workflow these days. cc @yzh119