[bugfix] Use FUSED_MC2 MoE comm path for the op dispatch_ffn_combine (#5156)

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

- Renames the MoE comm enum value `MoECommType.FUSED_ALLTOALL` to
`MoECommType.FUSED_MC2` and updates all call sites.
- Updates `select_moe_comm_method` to optionally select `FUSED_MC2` on
Ascend A3 when:
  - `enable_expert_parallel=True`
  - quantization is `w8a8_dynamic`
  - `EP <= 16`
  - `dynamic_eplb` is disabled
  - `is_mtp_model = False`
- Replaces the old “fused all-to-all” comm implementation with
`FusedMC2CommImpl`, using `TokenDispatcherWithMC2` /
`PrepareAndFinalizeWithMC2` and `dispatch_ffn_combine`.

- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

---------

Signed-off-by: Chen Chen <0109chenchen@gmail.com>

vllm_ascend/ascend_forward_context.py

@@ -26,7 +26,7 @@ class MoECommType(Enum):
     ALLGATHER = 0
     MC2 = 1
     ALLTOALL = 2
-    FUSED_ALLTOALL = 3
+    FUSED_MC2 = 3
 
 
 @contextmanager
@@ -62,11 +62,8 @@ def set_ascend_forward_context(
 
         from vllm_ascend.ops.fused_moe.moe_comm_method import \
             get_moe_comm_method
-        moe_comm_type = select_moe_comm_method(num_tokens, vllm_config)
-        # TODO: remove this after moe_comm_type selection logic is finalized
-        if is_mtp_model:
-            moe_comm_type = (MoECommType.ALLTOALL if moe_comm_type
-                             == MoECommType.FUSED_ALLTOALL else moe_comm_type)
+        moe_comm_type = select_moe_comm_method(num_tokens, vllm_config,
+                                               is_mtp_model)
         forward_context.moe_comm_type = moe_comm_type
         forward_context.moe_comm_method = get_moe_comm_method(moe_comm_type)
 
@@ -93,7 +90,7 @@ def set_ascend_forward_context(
         forward_context.mmrs_fusion = mmrs_fusion
         forward_context.num_tokens = num_tokens
         forward_context.sp_enabled = sp_enabled
-        #TODO(Levi-JQ): another PR to normalize the enabling logic for sp/fc2
+        # TODO(Levi-JQ): another PR to normalize the enabling logic for sp/fc2
         forward_context.flashcomm_v2_enabled = flashcomm2_enable(
         ) and tp_world_size > 1 and num_tokens is not None
 
@@ -210,29 +207,30 @@ def get_mc2_mask():
 
 
 def select_moe_comm_method(num_tokens: int,
-                           vllm_config: VllmConfig) -> Optional[MoECommType]:
-    """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.
+                           vllm_config: VllmConfig,
+                           is_mtp_model=False) -> Optional[MoECommType]:
+    """Select the MoE communication method according to parallel settings,
+    device generation, token count, and quantization.
 
-            a. For A2, we choose from MC2 and all-gather.
+    1. Non-MoE models return `None`.
+    2. Without expert parallel, fall back to all-gather.
+    3. On A2 with expert parallel, pick MC2 when tokens fit the MC2 capacity
+       and the DP size is large enough; otherwise use all-gather.
+    4. On A3 with expert parallel, prefer fused MC2 when using w8a8_dynamic
+       quantization with small EP size, no dynamic_eplb, and not in MTP
+       mode; otherwise use MC2 within capacity or all-to-all.
 
-            b. For A3, we choose from MC2 and all-to-all.
+    Args:
+        num_tokens (int): The number of tokens in the current batch.
+        vllm_config (VllmConfig): Runtime configuration for the model.
+        is_mtp_model (bool): Whether the model runs in MTP mode (disables fused MC2).
 
-            In both cases, we use MC2 when the number of tokens is smaller than
-            a its capacity threshold.
+    Raises:
+        ValueError: If the soc version is unsupported.
 
-        Args:
-            num_tokens (int): The number of tokens in the current batch.
-
-        Raises:
-            ValueError: If the soc version is unsupported.
-
-        Returns:
-            MoECommType: The selected MoE communication method.
-        """
+    Returns:
+        MoECommType | None: The selected MoE communication method.
+    """
     if not is_moe_model(vllm_config):
         return None
     mc2_tokens_capacity = get_mc2_tokens_capacity()
@@ -255,11 +253,13 @@ def select_moe_comm_method(num_tokens: int,
         ascend_config = get_ascend_config()
         dynamic_eplb = ascend_config.dynamic_eplb or ascend_config.expert_map_record_path
         # TODO: drop the EP-size guard when dispatch_ffn_combine supports larger EP sizes
-        fused_all2all_enable = quant_type == "w8a8_dynamic" and get_ep_group(
-        ).world_size <= 16 and (not dynamic_eplb)
-        moe_comm_type = (MoECommType.MC2 if num_tokens <= mc2_tokens_capacity
-                         else MoECommType.FUSED_ALLTOALL
-                         if fused_all2all_enable else MoECommType.ALLTOALL)
+        fused_mc2_enable = envs_ascend.VLLM_ASCEND_ENABLE_FUSED_MC2 and quant_type == "w8a8_dynamic" and get_ep_group(
+        ).world_size <= 16 and (not dynamic_eplb) and (not is_mtp_model)
+        if num_tokens <= mc2_tokens_capacity:
+            moe_comm_type = MoECommType.FUSED_MC2 if fused_mc2_enable else MoECommType.MC2
+        else:
+            moe_comm_type = MoECommType.FUSED_MC2 if fused_mc2_enable else MoECommType.ALLTOALL
+
     else:
         raise ValueError(f"Unsupported soc_version: {soc_version}")
     return moe_comm_type