[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/envs.py

@@ -132,6 +132,9 @@ env_variables: Dict[str, Callable[[], Any]] = {
     # Whether to anbale dynamic EPLB
     "DYNAMIC_EPLB":
     lambda: os.getenv("DYNAMIC_EPLB", "false").lower(),
+    # Whether to anbale fused mc2(dispatch_gmm_combine_decode/dispatch_ffn_combine operator)
+    "VLLM_ASCEND_ENABLE_FUSED_MC2":
+    lambda: int(os.getenv("VLLM_ASCEND_ENABLE_FUSED_MC2", '0')),
 }
 
 # end-env-vars-definition