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

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

vllm/model_executor/layers/mamba/mamba_mixer.py

@@ -1,7 +1,10 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
-from typing import NamedTuple, Optional
+from typing import TYPE_CHECKING, NamedTuple, Optional
+
+if TYPE_CHECKING:
+    from vllm.attention.backends.abstract import AttentionBackend
 
 import torch
 from torch import nn
@@ -404,6 +407,11 @@ class MambaMixer(MambaBase, CustomOp):
     def mamba_type(self) -> str:
         return "mamba1"
 
+    def get_attn_backend(self) -> type["AttentionBackend"]:
+        from vllm.v1.attention.backends.mamba1_attn import (
+            Mamba1AttentionBackend)
+        return Mamba1AttentionBackend
+
     def _time_proj_bias(self) -> Optional[torch.Tensor]:
         if hasattr(self.dt_proj, "bias") and self.dt_proj.bias is not None:
             return self.dt_proj.bias.float()