[2/N][Feat] Add MC2 communication method for MoE layers (#2469)

### What this PR does / why we need it?
This method replaces the previous all-gather approach for small numbers
of tokens.

The key changes include:
- A new `AscendFusedMoE` layer that handles token splitting, local
computation, and final aggregation via all-gather.
- Logic in the model runner to dynamically select between the new MC2
method and the existing all-gather method based on the number of input
tokens.
- Sharding the MoE communication mask across tensor-parallel ranks.

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

### How was this patch tested?
Test case fixed.


- vLLM version: v0.10.1.1
- vLLM main:
b00e69f8ca

---------

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

vllm_ascend/utils.py

@@ -509,6 +509,9 @@ def register_ascend_customop():
     from vllm_ascend.ops.layernorm import AscendRMSNorm
     CustomOp.register_oot(_decorated_op_cls=AscendRMSNorm, name="RMSNorm")
 
+    from vllm_ascend.ops.common_fused_moe import AscendFusedMoE
+    CustomOp.register_oot(_decorated_op_cls=AscendFusedMoE, name="FusedMoE")
+
     # NOTE: Keep this at last to ensure all custom actions are registered
     _ASCEND_CUSTOMOP_IS_REIGISTERED = True