[Attention] Unify mamba and attention backend selection (#23171)

Signed-off-by: Ayush Satyam <ayushsatyam146@gmail.com>

vllm/model_executor/layers/mamba/mamba_mixer2.py

@@ -1,7 +1,10 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
-from typing import Optional, Union
+from typing import TYPE_CHECKING, Optional, Union
+
+if TYPE_CHECKING:
+    from vllm.attention.backends.abstract import AttentionBackend
 
 import torch
 from torch import nn
@@ -758,6 +761,11 @@ class MambaMixer2(MambaBase, CustomOp):
     def mamba_type(self) -> str:
         return "mamba2"
 
+    def get_attn_backend(self) -> type["AttentionBackend"]:
+        from vllm.v1.attention.backends.mamba2_attn import (
+            Mamba2AttentionBackend)
+        return Mamba2AttentionBackend
+
 
 def mamba_mixer2(
     hidden_states: torch.Tensor,