[AMD] Fix multimodal diffusion test crash on ROCm by falling back to SDPA (#22335)

python/sglang/jit_kernel/flash_attention_v3.py

@@ -78,6 +78,8 @@ def _is_fa3_supported(device=None) -> bool:
     #  https://docs.nvidia.com/cuda/cuda-c-programming-guide/#shared-memory-8-x
     #  And for sgl-kernel right now, we can build fa3 on sm80/sm86/sm89/sm90a.
     #  That means if you use A100/A*0/L20/L40/L40s/4090 you can use fa3.
+    if torch.version.cuda is None:
+        return False
     return (torch.version.cuda >= "12.3") and (
         torch.cuda.get_device_capability(device)[0] == 9
         or torch.cuda.get_device_capability(device)[0] == 8

python/sglang/multimodal_gen/runtime/platforms/rocm.py

@@ -149,10 +149,19 @@ class RocmPlatform(Platform):
             try:
                 import flash_attn  # noqa: F401
 
+                from sglang.jit_kernel.flash_attention_v3 import _is_fa3_supported
                 from sglang.multimodal_gen.runtime.layers.attention.backends.flash_attn import (  # noqa: F401
                     FlashAttentionBackend,
                 )
 
+                if not _is_fa3_supported():
+                    logger.info(
+                        "FlashAttention backend now dispatches through FA3 "
+                        "(CUDA-only). Using Torch SDPA backend on ROCm."
+                    )
+                    target_backend = AttentionBackendEnum.TORCH_SDPA
+
+                if target_backend == AttentionBackendEnum.FA:
                     supported_sizes = FlashAttentionBackend.get_supported_head_sizes()
                     if head_size not in supported_sizes:
                         logger.info(