bugfix for mtp with multistream_moe (#3419)

### What this PR does / why we need it?
when infer deepseek mtp layer with multistream_moe, we should pass a
boolean to evaluate this feature and fix bugs when we are in mtp layer

- vLLM version: v0.11.0rc3
- vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.0

---------

Signed-off-by: zouyida2052 <zouyida2002@gmail.com>

tests/e2e/singlecard/spec_decode_v1/test_v1_mtp_torchair_correctness.py

@@ -41,6 +41,7 @@ def test_mtp_torchair_correctness(
                             "use_cached_graph": False,
                             "graph_batch_sizes": [1, 2, 4],
                         },
+                        "multistream_overlap_shared_expert": "True"
                     }) as ref_llm:
         ref_outputs = ref_llm.generate(example_prompts, sampling_config)
     with VllmRunner(model_name,
@@ -60,7 +61,8 @@ def test_mtp_torchair_correctness(
                             "enabled": True,
                             "use_cached_graph": False,
                             "graph_batch_sizes": [1, 2, 4],
-                        }
+                        },
+                        "multistream_overlap_shared_expert": "True"
                     }) as spec_llm:
         spec_outputs = spec_llm.generate(example_prompts, sampling_config)