CUDA/HIP: Flash Attention tuning (gfx1201) (#28102)
* HIP: enable mma FA for head size 256 on RDNA4, tune configs
Assisted-by: Claude
Assisted-by: Codex
* HIP: prefer whole-tile FA grids over stream-k on AMD WMMA
Assisted-by: Claude
Assisted-by: Codex
* revise stream_k logic
* revise kernel selection logic
---------
Co-authored-by: Johannes Gäßler <johannesg@5d6.de>
