vulkan: force full subgroups for flash attention to fix intel subgroup crash

[0cc4m]

Fixes #17297

To use subgroup operations on Intel, we have to force full subgroups, otherwise the subgroup size and how many of the threads are actually active may vary. I don't think this change should have any negative effect on other drivers.

[jeffbolznv]

While I agree this is worth a try, I don't understand why the failing change actually needs it. It doesn't assume a specific subgroup size or mapping of invocations to subgroups.

[0cc4m]

I don't fully understand it, but I think if VK_PIPELINE_SHADER_STAGE_CREATE_REQUIRE_FULL_SUBGROUPS_BIT is not set, a Vulkan driver may disable specific threads of a subgroup for performance reasons, which leads to undefined behaviour when subgroup operations are used.

I can't find much information about this in the specification, it's mostly experience from dealing with Intel GPUs which do vary their subgroup size. I guess someone would need to dig deeper into the ANV driver to figure out what exactly it is doing.