[Target] Add a default warp size 1 for vulkan and opencl

[masahi]

I've made this change too many times during my development, to workaround topi scatter op that requires thread_warp_size for no good reason. Since #8030 transpose op also requires thread_warp_size and all models that use transpose op are broken on vk.

@tmoreau89 @Lunderberg @junrushao1994 @zxybazh

[zxybazh]

The code looks good to me. May I know why the default thread_warp_size is 1 here?

[masahi]

Because it need to support ALL vulkan or opencl devices. 1 is the only value that could work, but it is useless in terms of performance. It's only there so that compiling models on vulkan wouldn't break.

[zxybazh]

Thanks for the quick reply. In that case, thread_warp_size of 1 should be a good default value.

[Lunderberg]

Looks good to me, and I agree on using 1 as the default thread_warp_size. Both the VkPhysicalDeviceSubgroupProperties and the VkPhysicalDeviceSubgroupSizeControlPropertiesEXT documentation give a minimum subgroup size of 1, so the spec doesn't give us any further guarantees.