cuda : fix 2-bit quants on amd hip

[Engininja2]

Fixes evaluating iq2_xxs and iq2_xs on AMD HIP from HIP's half2 components appearing to be shorts instead of half. main would generate '######' and some MUL_MAT_ID and MUL_MAT ops would fail.

[JohannesGaessler]

There are functions __low2half and __high2half that do the exact same thing as .x and .y on NVIDIA but notably also do the correct thing on AMD. I suggest you use those instead; check the rest of the code for examples.

[Artefact2]

Thanks for this! I can confirm this works on gfx1030 on Linux/rocm 5.7.1.

[sorasoras]

Tested on windows with rocm 5.7.1.
I can confirm working on my 7900XTX

[Engininja2]

I used __low2float since the value was immediately being cast to float and that function is already used elsewhere in the code.