[BUGFIX][ROCM] ViT FlashAttention on ROCm (no GFX9) and contiguous on qwen3vl ROCm TORCH_SDPA (#27190)

Signed-off-by: JartX <sagformas@epdcenter.es>
Co-authored-by: tjtanaa <tunjian.tan@embeddedllm.com>

vllm/platforms/rocm.py

@@ -205,12 +205,16 @@ class RocmPlatform(Platform):
 
     @classmethod
     def get_vit_attn_backend(cls, head_size: int, dtype: torch.dtype) -> "_Backend":
+        from importlib.util import find_spec
+
         from vllm.attention.backends.registry import _Backend
 
         if envs.VLLM_ROCM_USE_AITER and envs.VLLM_ROCM_USE_AITER_MHA and on_gfx9():
             return _Backend.ROCM_AITER_FA
-        if on_gfx9():
+
+        if on_gfx9() and find_spec("flash_attn") is not None:
             return _Backend.FLASH_ATTN
+
         return _Backend.TORCH_SDPA
 
     @classmethod