[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>

vllm_ascend/worker/model_runner_v1.py

@@ -1434,14 +1434,39 @@ class NPUModelRunner(LoRAModelRunnerMixin):
         )
 
     def _select_moe_comm_method(self, num_tokens: int) -> str:
+        """1. If expert parallel is not enabled, we use all-gather since MC2 and all-to-all
+        are designed for expert parallelism.
+        2. If expert parallel is enabled, we need to consider the soc version and the
+        number of tokens. This is based on the observation that all-gather is more
+        efficient than all-to-all when running on A2.
+            
+            a. For A2, we choose from MC2 and all-gather.
+            
+            b. For A3, we choose from MC2 and all-to-all.
+            
+            In both cases, we use MC2 when the number of tokens is smaller than
+            a its capacity threshold.
+
+        Args:
+            num_tokens (int): The number of tokens in the current batch.
+
+        Raises:
+            ValueError: If the soc version is unsupported.
+
+        Returns:
+            str: The selected MoE communication method, either "allgather", "mc2", or "alltoall".
+        """
         soc_version = get_ascend_soc_version()
 
-        if num_tokens <= self.mc2_tokens_capacity:
-            moe_comm_method = "mc2"
-        elif soc_version in {AscendSocVersion.A2}:
+        if not self.parallel_config.enable_expert_parallel:
             moe_comm_method = "allgather"
+        elif soc_version in {AscendSocVersion.A2}:
+            if num_tokens <= self.mc2_tokens_capacity and self.parallel_config.world_size >= 16:
+                moe_comm_method = "mc2"
+            else:
+                moe_comm_method = "allgather"
         elif soc_version in {AscendSocVersion.A3}:
-            moe_comm_method = "alltoall"
+            moe_comm_method = "mc2" if num_tokens <= self.mc2_tokens_capacity else "alltoall"
         else:
             raise ValueError(f"Unsupported soc_version: {soc_version}")