[Bugfix][MM encoder] Fix ViT attention backend resolving for Turing GPU (#29614)

Signed-off-by: Isotr0py <mozf@mail2.sysu.edu.cn>

vllm/platforms/cuda.py

@@ -264,14 +264,15 @@ class CudaPlatformBase(Platform):
         cls, head_size: int, dtype: torch.dtype
     ) -> "AttentionBackendEnum":
         # Try FlashAttention first
-        try:
-            backend_class = AttentionBackendEnum.FLASH_ATTN.get_class()
-            if backend_class.supports_head_size(
-                head_size
-            ) and backend_class.supports_dtype(dtype):
-                return AttentionBackendEnum.FLASH_ATTN
-        except ImportError:
-            pass
+        if (cc := cls.get_device_capability()) and cc.major >= 8:
+            try:
+                backend_class = AttentionBackendEnum.FLASH_ATTN.get_class()
+                if backend_class.supports_head_size(
+                    head_size
+                ) and backend_class.supports_dtype(dtype):
+                    return AttentionBackendEnum.FLASH_ATTN
+            except ImportError:
+                pass
 
         return AttentionBackendEnum.TORCH_SDPA