[Model][Mamba] Add selector for mamba attention backend and make it pluggable for other device (#26487)

Signed-off-by: shen-shanshan <467638484@qq.com>

vllm/model_executor/layers/mamba/mamba_mixer2.py

@@ -1,10 +1,6 @@
 # SPDX-License-Identifier: Apache-2.0
 # SPDX-FileCopyrightText: Copyright contributors to the vLLM project
 
-from typing import TYPE_CHECKING
-
-if TYPE_CHECKING:
-    from vllm.attention.backends.abstract import AttentionBackend
 
 import torch
 from torch import nn
@@ -908,11 +904,6 @@ 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(
     projected_states: torch.Tensor,