[Bugfix] Fix Qwen2.5-Omni-7B accuarcy test (#4556)

### What this PR does / why we need it?
Fix Qwen2.5-Omni-7B accuarcy test
issue：https://github.com/vllm-project/vllm-ascend/issues/4480
Depends on : https://github.com/vllm-project/vllm-ascend/pull/4534

- vLLM version: v0.11.2
- vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.2

Signed-off-by: hfadzxy <starmoon_zhang@163.com>

vllm_ascend/ops/register_custom_ops.py

@@ -173,7 +173,9 @@ def _maybe_prefetch_mlp_down_proj_impl(x_dependency: torch.Tensor) -> None:
     except AssertionError:
         return
 
-    if not forward_context.prefetch_mlp_enabled:
+    prefetch_mlp_enabled = getattr(forward_context, 'prefetch_mlp_enabled',
+                                   False)
+    if not prefetch_mlp_enabled:
         return
     forward_context.prefetch_mlp_down_proj = True
     model_instance = forward_context.model_instance
@@ -202,7 +204,9 @@ def _maybe_wait_prefetch_done_impl(x: torch.Tensor) -> None:
     except AssertionError:
         return
 
-    if not forward_context.prefetch_mlp_enabled:
+    prefetch_mlp_enabled = getattr(forward_context, 'prefetch_mlp_enabled',
+                                   False)
+    if not prefetch_mlp_enabled:
         return
     if forward_context.prefetch_mlp_gate_up_proj or \
         forward_context.prefetch_mlp_down_proj: