[attn][tiny fix] fix attn backend in MultiHeadAttention (#11463)

Signed-off-by: Mengqing Cao <cmq0113@163.com>

vllm/attention/layer.py

@@ -191,6 +191,7 @@ class MultiHeadAttention(nn.Module):
                                         kv_cache_dtype=None,
                                         block_size=16,
                                         is_attention_free=False)
+        attn_backend = backend_name_to_enum(attn_backend.get_name())
         if attn_backend in {_Backend.FLASH_ATTN, _Backend.FLASH_ATTN_VLLM_V1}:
             attn_backend = _Backend.XFORMERS