[FA2 performance] flashattention with dim=128 get ~90% of xetla

[Dewei-Wang-sh]

currently on the main branch, we get ~85%.

[Dewei-Wang-sh]

comparing the assembly with XeTLA, trying to find the key difference.

[Dewei-Wang-sh]

auto tuning helps improve from 39to 44 tflops for the 4x48x1024x128 case