[Feat] Flash comm allgher ep (#3334)

Support flash comm v1(Sequence Parallelism) for Allgather EP.

- vLLM version: v0.11.0rc3
- vLLM main: https://github.com/vllm-project/vllm/commit/v0.11.0

---------

Signed-off-by: realliujiaxu <realliujiaxu@163.com>
Co-authored-by: zhaozx-cn <zhaozx2116@163.com>

tests/ut/attention/test_mla_v1.py

@@ -500,9 +500,12 @@ class TestAscendMLAImpl(TestBase):
         mock_up_proj.assert_called_once()
         mock_npu_fused_infer_attention_score.assert_called_once()
 
+    @patch("torch.ops.vllm.maybe_all_gather_and_maybe_unpad")
     @patch("vllm_ascend.attention.mla_v1.maybe_npu_prefetch")
-    def test_mla_preprocess(self, magic_npu_fetch):
+    def test_mla_preprocess(self, magic_npu_fetch,
+                            mock_maybe_all_gather_and_maybe_unpad):
         magic_npu_fetch.return_value = MagicMock()
+        mock_maybe_all_gather_and_maybe_unpad.side_effect = lambda x, label: x
         batch_size = 4
         seq_len = 8
         hidden_size = 1024