Re-implement FlashAttention with new Xe atoms

[petercad]

This PR updates FlashAttention to the new copy/MMA atoms.

Changes:

• Prefill and decode unified into a single implementation, allowing simultaneous K and Q subgroup-level parallelization rather than an either-or.
• GEMMs and softmax grouped together and the full k loop consolidated into an FMHA mainloop class.
  • This will facilitate further manual pipelining/overlap of GEMM with softmax.
• Use new copy/MMA atoms and reorders to transparently support arbitrary data types.
• Automatic copy/MMA operator selection.

Current status: prefill/decode examples almost all working, similar/better performance to old examples.

Known issues:

• Head size 192 decode config doesn't compile yet -- to be fixed.
• Strange SYCL compiler behavior/bug with tSrS->tArP reorder. Apparently the compiler believes there is UB somewhere and will omit a large section of the kernel as a result. For the moment, there's a direct copy as a workaround while I pin down the issue. I'm not able to reproduce this behavior with the reorder in isolation.

Additional features (causal masking, variable sequence lengths, etc.) to be added later.

[petercad]

I will break up this large commit into self-contained smaller commits after review is complete.