[Bugfix]: Fix cross attention backend selection for Turing GPU (#31806)

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

vllm/attention/layers/cross_attention.py

@@ -149,16 +149,20 @@ class CrossAttention(Attention):
             kv_cache_dtype = "auto"
             block_size = 16
 
-        underlying_attn_backend = get_attn_backend(
-            head_size, dtype, kv_cache_dtype, block_size
-        )
-        attn_backend = create_cross_attention_backend(underlying_attn_backend)
-
         if attn_type is not None:
             assert attn_type == AttentionType.ENCODER_DECODER, (
                 "CrossAttention only supports AttentionType.ENCODER_DECODER"
             )
 
+        underlying_attn_backend = get_attn_backend(
+            head_size,
+            dtype,
+            kv_cache_dtype,
+            block_size,
+            attn_type=AttentionType.ENCODER_DECODER,
+        )
+        attn_backend = create_cross_attention_backend(underlying_attn_backend)
+
         super().__init__(
             num_heads=num_heads,
             head_size=head_size,