add gatherep select. (#2740)

### What this PR does / why we need it?
add gatherep select.

- vLLM version: v0.10.1.1
- vLLM main:
e599e2c65e

Signed-off-by: wangxiaoxin-sherie <wangxiaoxin7@huawei.com>
Co-authored-by: wangxiaoxin-sherie <wangxiaoxin7@huawei.com>

tests/ut/ops/test_ascend_forwad_context.py

@@ -0,0 +1,22 @@
+import os
+import unittest
+from unittest import mock
+
+from vllm_ascend.ascend_forward_context import get_dispatcher_name
+
+
+class TestGetDispatcherName(unittest.TestCase):
+
+    def test_get_dispatcher_name(self):
+        result = get_dispatcher_name(1, False)
+        assert result == "TokenDispatcherWithAllGather"
+        result = get_dispatcher_name(4, False)
+        assert result == "TokenDispatcherWithAll2AllV"
+        result = get_dispatcher_name(16, True)
+        assert result == "TokenDispatcherWithAll2AllV"
+        result = get_dispatcher_name(16, False)
+        assert result == "TokenDispatcherWithMC2"
+        with mock.patch.dict(os.environ,
+                             {"VLLM_ENABLE_FUSED_EXPERTS_ALLGATHER_EP": "1"}):
+            result = get_dispatcher_name(16, False)
+            assert result == "TokenDispatcherWithAllGather"