[Bugfix] Fix FusedMoEPrepareAndFinalize for cuda-disalike backends (vllm-project#18178)

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

Author: MengqingCao

vllm/model_executor/layers/fused_moe/layer.py

@@ -43,6 +43,7 @@
  from .pplx_prepare_finalize import PplxPrepareAndFinalize
 else:
  fused_experts = None # type: ignore
+ FusedMoEPrepareAndFinalize = None # type: ignore
 if is_rocm_aiter_moe_enabled():
  from vllm.model_executor.layers.fused_moe.rocm_aiter_fused_moe import ( # noqa: E501
  rocm_aiter_biased_group_topk as grouped_topk)