[Refactor][MoE] Reuse vLLM's all_reduce logic (#5189)

### What this PR does / why we need it?
Move all_reduce logic to AscendFusedMoE.forward, reuse vLLM's logic.

### Does this PR introduce _any_ user-facing change?
No

### How was this patch tested?
e2e & ut
- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

Signed-off-by: weichen <calvin_zhu0210@outlook.com>
Co-authored-by: weijinqian0 <1184188277@qq.com>

tests/ut/ops/test_prepare_finalize.py

@@ -169,15 +169,12 @@ class TestPrepareAndFinalize(unittest.TestCase):
         self.assertEqual(final_result.shape[0], 2)
 
     @patch("vllm_ascend.ops.fused_moe.prepare_finalize.get_dp_group")
-    @patch(
-        "vllm_ascend.ops.fused_moe.prepare_finalize.tensor_model_parallel_all_reduce"
-    )
     @patch("vllm_ascend.ops.fused_moe.prepare_finalize.get_forward_context")
     @patch("vllm_ascend.ops.fused_moe.prepare_finalize.enable_sp",
            return_value=False)
     def test_allgather_prepare_finalize(self, mock_enable_sp,
                                         mock_get_forward_context,
-                                        mock_tp_all_reduce, mock_get_dp_group):
+                                        mock_get_dp_group):
         # Mock forward context
         mock_context = MagicMock()
         mock_context.max_tokens_across_dp = 6
@@ -222,7 +219,5 @@ class TestPrepareAndFinalize(unittest.TestCase):
 
         self.assertEqual(result.shape[0], 3)
 
-        # Test with TP all-reduce
-        mock_tp_all_reduce.return_value = result
         result_with_tp = layer.finalize(h_out, reduce_results=True)
         self.assertEqual(result_with_tp.shape[0], 3)