[BugFix] Require kv producer for layer sharding (#8563)

### What this PR does / why we need it?

This PR introduce stricter Ascend `additional_config.layer_sharding`
validation to the 0.18 release branch so it is only accepted on
PD-disaggregated P nodes with `kv_role="kv_producer"`.


### Does this PR introduce _any_ user-facing change?

No.

### How was this patch tested?

E2E test

---------

Signed-off-by: chenchuw886 <chenchuw@huawei.com>
Co-authored-by: chenchuw886 <chenchuw@huawei.com>

tests/e2e/multicard/2-cards/test_offline_inference_distributed.py

@@ -26,6 +26,7 @@ from unittest.mock import patch
 
 import pytest
 from vllm import SamplingParams
+from vllm.config import KVTransferConfig
 
 from tests.e2e.conftest import VllmRunner, wait_until_npu_memory_free
 from tests.e2e.model_utils import check_outputs_equal
@@ -180,6 +181,7 @@ def test_qwen3_moe_fc2_oshard_tp2() -> None:
         enable_expert_parallel=True,
         enforce_eager=True,  # TODO(Levi-JQ): support graph mode for fc2 in Qwen
         additional_config={"layer_sharding": ["o_proj"]},
+        kv_transfer_config=KVTransferConfig(kv_role="kv_producer"),
     ) as vllm_model:
         vllm_model.generate(example_prompts, sampling_params)
 
@@ -261,7 +263,6 @@ def test_deepseek3_2_w8a8_pruning_mtp_tp2_ep():
         max_model_len=163840,
         compilation_config={"cudagraph_capture_sizes": [2, 4, 6, 8, 10, 12], "cudagraph_mode": "FULL_DECODE_ONLY"},
         speculative_config={"num_speculative_tokens": 1, "method": "deepseek_mtp"},
-        additional_config={"layer_sharding": ["q_b_proj", "o_proj"]},
         reasoning_parser="deepseek_v3",
         tokenizer_mode="deepseek_v32",
         gpu_memory_utilization=0.8,
@@ -290,7 +291,7 @@ def test_deepseek3_2_w8a8c8_pruning_mtp_tp2_ep():
         max_model_len=163840,
         compilation_config={"cudagraph_capture_sizes": [2, 4, 6, 8, 10, 12], "cudagraph_mode": "FULL_DECODE_ONLY"},
         speculative_config={"num_speculative_tokens": 1, "method": "deepseek_mtp"},
-        additional_config={"layer_sharding": ["q_b_proj", "o_proj"], "enable_sparse_c8": True},
+        additional_config={"enable_sparse_c8": True},
         reasoning_parser="deepseek_v3",
         tokenizer_mode="deepseek_v32",
         gpu_memory_utilization=0.8,