[Fix][MoE] Refine MoE communication strategy (#2734)

### What this PR does / why we need it?
Refactors the Mixture-of-Experts (MoE) communication method selection
logic. The choice between all-gather, all-to-all, and mc2 is now
determined by expert parallel configuration, SoC version (A2/A3), and
token count for better performance.

### Does this PR introduce _any_ user-facing change?
None.

### How was this patch tested?
Added.


- vLLM version: v0.10.1.1
- vLLM main:
eafa8dcde6

---------

Signed-off-by: Yizhou Liu <liu_yizhou@outlook.com>

tests/ut/worker/test_model_runner_v1.py

@@ -0,0 +1,94 @@
+# Licensed under the Apache License, Version 2.0 (the "License");
+# you may not use this file except in compliance with the License.
+# You may obtain a copy of the License at
+#
+#     http://www.apache.org/licenses/LICENSE-2.0
+#
+# Unless required by applicable law or agreed to in writing, software
+# distributed under the License is distributed on an "AS IS" BASIS,
+# WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+# See the License for the specific language governing permissions and
+# limitations under the License.
+# This file is a part of the vllm-ascend project.
+
+from unittest.mock import MagicMock, patch
+
+import pytest
+
+from vllm_ascend.utils import AscendSocVersion
+from vllm_ascend.worker.model_runner_v1 import NPUModelRunner
+
+
+# yapf: disable
+@pytest.mark.parametrize(
+    "soc_version, enable_expert_parallel, world_size, num_tokens, mc2_tokens_capacity, expected_method",
+    [
+        # Case 1: Expert parallel is disabled, should always be 'allgather'
+        (AscendSocVersion.A2, False, 8, 100, 256, "allgather"),
+        (AscendSocVersion.A3, False, 16, 500, 256, "allgather"),
+
+        # Case 2: A2 SOC
+        # 2.1: MC2 conditions met (tokens <= capacity, world_size >= 16)
+        (AscendSocVersion.A2, True, 16, 100, 256, "mc2"),
+        (AscendSocVersion.A2, True, 32, 256, 256, "mc2"),
+        # 2.2: MC2 token capacity exceeded
+        (AscendSocVersion.A2, True, 16, 257, 256, "allgather"),
+        # 2.3: MC2 world size not met
+        (AscendSocVersion.A2, True, 8, 100, 256, "allgather"),
+        (AscendSocVersion.A2, True, 15, 100, 256, "allgather"),
+
+        # Case 3: A3 SOC
+        # 3.1: MC2 condition met (tokens <= capacity)
+        (AscendSocVersion.A3, True, 8, 100, 256, "mc2"),
+        (AscendSocVersion.A3, True, 16, 256, 256, "mc2"),
+        # 3.2: MC2 token capacity exceeded
+        (AscendSocVersion.A3, True, 8, 257, 256, "alltoall"),
+        (AscendSocVersion.A3, True, 16, 500, 256, "alltoall"),
+
+    ])
+# yapf: enable
+def test_select_moe_comm_method(soc_version, enable_expert_parallel,
+                                world_size, num_tokens, mc2_tokens_capacity,
+                                expected_method):
+    """
+    Tests the _select_moe_comm_method with various configurations.
+    """
+    # Mock the NPUModelRunner instance and its dependencies
+    mock_runner = MagicMock(spec=NPUModelRunner)
+    mock_runner.parallel_config = MagicMock()
+    mock_runner.parallel_config.enable_expert_parallel = enable_expert_parallel
+    mock_runner.parallel_config.world_size = world_size
+    mock_runner.mc2_tokens_capacity = mc2_tokens_capacity
+
+    # Patch the helper functions
+    with patch('vllm_ascend.worker.model_runner_v1.get_ascend_soc_version',
+               return_value=soc_version), \
+         patch('vllm_ascend.worker.model_runner_v1.is_global_first_rank',
+               return_value=True):
+
+        # Call the method under test
+        method = NPUModelRunner._select_moe_comm_method(
+            mock_runner, num_tokens)
+
+        # Assert the result
+        assert method == expected_method
+
+
+def test_select_moe_comm_method_unsupported_soc():
+    """
+    Tests that _select_moe_comm_method raises ValueError for an unsupported SOC.
+    """
+    mock_runner = MagicMock(spec=NPUModelRunner)
+    mock_runner.parallel_config = MagicMock()
+    mock_runner.parallel_config.enable_expert_parallel = True
+    mock_runner.mc2_tokens_capacity = 256
+
+    unsupported_soc = "UnsupportedSOC"
+
+    with patch('vllm_ascend.worker.model_runner_v1.get_ascend_soc_version',
+               return_value=unsupported_soc), \
+         patch('vllm_ascend.worker.model_runner_v1.is_global_first_rank',
+               return_value=True), \
+         pytest.raises(ValueError, match=f"Unsupported soc_version: {unsupported_soc}"):
+
+        NPUModelRunner._select_moe_comm_method(mock_runner, 100)