Optimizations for Decode Attn fp8 kernel MI350

[mycpuorg]

Optimizations for Decode Attn fp8 kernel

 - Baseline (MI350 FP8 split‑K forward)
      - Runtime ~125 µs (BF16 path was 110 µs).
      - rocprof showed the Triton JIT emitted a 512‑VGPR kernel with 56 spills, so each SIMD carried only a single wavefront.
      - The inner loop kept every K/V fragment plus dequant buffers resident, and the HIP autotuner could pick even heavier configs (≥150 VGPR), cementing the one‑wave bottleneck.
  - Streaming K and On‑Demand V ( xformers/ops/fmha/_triton/splitk_kernels.py )
      - Rewrote the FP8 branch so each quantization group loads K directly into the dot product and reloads V only when updating the accumulator.
      - Removed the persistent register lists for K/V, collapsing VGPR usage to 108 with zero spills; MI350 can now run two waves per SIMD.
      - Result: FP8 runtime dropped to ~105 µs, now beating BF16’s 110 µs.
  - HIP Autotune Guardrails (same file)
      - Constrained the HIP autotuner to tiles ≤64×64 and ≤4 warps, preventing Triton from revisiting the high‑VGPR plans.
      - Ensures every new launch stays in the low‑register regime uncovered by the streaming change.
  - Forced HIP FP8 Launch Parameters ( xformers/ops/fmha/triton_splitk.py )
      - Added FwOp.force_kernel_config and, by default, return the measured best tuple (BLOCK_M=16, BLOCK_N=64, num_stages=2, num_warps=1) whenever FP8 scale/shift tensors are present.
      - Eliminates heuristics drifting at runtime and locks in the ~105 µs profile.

[mycpuorg]

dup of #74