CUDA: add CDNA3 MFMA support for flash attention MMA kernel (#19806)

* CUDA: add CDNA3 MFMA support for flash attention MMA kernel

Add MI300X (gfx942) MFMA tensor core flash attention using
v_mfma_f32_16x16x16_f16 (FP16 in, FP32 accumulate).

- Add FATTN_WARP_SIZE=64 for CDNA wavefront64
- Add CDNA config for head sizes 64, 80, 96, 112, 128
- Add FP16 MFMA intrinsic path in mma.cuh
- Add manual V transpose load for MFMA register layout
- Route CDNA to MMA for prompt processing, VEC for token generation
- Fix Q loading and combine stride granularity for non-power-of-2 heads

Benchmarks (Qwen2.5-1.5B Q4_K_M, MI300X):
  pp512  +7%,  pp1024 +13%,  pp2048 +23%,  pp4096 +39%
  tg128  -10% (FA overhead, VEC used for both)

All 2480 flash attention tests pass.

Ref: https://github.com/ggml-org/llama.cpp/issues/17917

* address review: replace FATTN_WARP_SIZE with constexpr, improve dispatch

- Replace #define FATTN_WARP_SIZE with constexpr int warp_size =
  ggml_cuda_get_physical_warp_size() in each device function
- Use ne[1]*gqa_ratio threshold for MMA vs tile dispatch. Benchmarked
  crossover on MI300X @ d32768 with power-of-2 GQA models:
    hsk=64  (Llama 1B, gqa=4): MMA wins at eff >= 128 (+11%)
    hsk=128 (Llama 3B, gqa=4): MMA wins at eff >= 128 (+4%)
  Unified threshold: eff_nq >= 128 for all head sizes.
- Remove VEC fallback; small batches fall through to tile kernel

* Update ggml/src/ggml-cuda/fattn.cu

* use ggml_cuda_info().devices warp_size instead of hardcoded check

---------

Co-authored-by: Johannes Gäßler <johannesg@5d6.de>

ggml/src/ggml-cuda/fattn.cu

@@ -440,6 +440,18 @@ static best_fattn_kernel ggml_cuda_get_best_fattn_kernel(const int device, const
         return BEST_FATTN_KERNEL_MMA_F16;
     }
 
+    // Use MFMA flash attention for CDNA (MI100+):
+    if (amd_mfma_available(cc) && Q->ne[0] != 40 && Q->ne[0] != 72 && Q->ne[0] != 256 && Q->ne[0] != 576) {
+        const int64_t eff_nq = Q->ne[1] * (gqa_opt_applies ? gqa_ratio : 1);
+        // MMA vs tile crossover benchmarked on MI300X @ d32768:
+        //   hsk=64  (gqa=4): MMA wins at eff >= 128 (+11%)
+        //   hsk=128 (gqa=4): MMA wins at eff >= 128 (+4%)
+        if (eff_nq >= (GGML_CUDA_CC_IS_CDNA1(cc) && Q->ne[0] == 64 ? 64 : 128)) {
+            return BEST_FATTN_KERNEL_MMA_F16;
+        }
+        // Fall through to tile kernel for small effective batch sizes.
+    }
+
     // If there are no tensor cores available, use the generic tile kernel:
     if (can_use_vector_kernel) {
         if (!ggml_is_quantized(K->type) && !ggml_is_quantized(V->type)) {