[UT]Ut for function cumsum_group_list in moe_mlp (ref #5025) (#5036)

### What this PR does / why we need it?
Add ut for the cumsum_group_list function, which is related to the
precision issues stemming from the moe_mlp.py .
The ralated PR is https://github.com/vllm-project/vllm-ascend/pull/5025

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

- vLLM version: v0.12.0
- vLLM main:
ad32e3e19c

---------

Signed-off-by: tanqingshan (A)  <50050625@china.huawei.com>
Co-authored-by: tanqingshan (A) <50050625@china.huawei.com>
Co-authored-by: wangxiyuan <wangxiyuan1007@gmail.com>
Co-authored-by: weijinqian0 <1184188277@qq.com>

tests/ut/ops/test_moe_mlp.py

@@ -0,0 +1,51 @@
+import unittest
+from typing import ClassVar
+
+import torch
+
+from vllm_ascend.ops.fused_moe.moe_mlp import cumsum_group_list
+
+
+class TestCumsumGroupList(unittest.TestCase):
+    glist_dict: ClassVar[dict[int, torch.Tensor]]
+
+    @classmethod
+    def setUpClass(cls):
+        cls.glist_dict = {
+            0: torch.tensor([0, 2, 3, 3]),
+            1: torch.tensor([0, 2, 1, 0]),
+            2: torch.tensor([[1, 2], [2, 1], [0, 0], [0, 0]])
+        }
+
+    support_combine = [(0, 0), (1, 0), (0, 1)]
+    unsupport_combine = [(0, 2), (2, 1), (1, 2)]
+
+    def test_cumsum_group_list_supported_conversion(self):
+        for src_list_type, dst_list_type in self.support_combine:
+            with self.subTest(src=src_list_type, dst=dst_list_type):
+                result = cumsum_group_list(self.glist_dict[src_list_type],
+                                           src_list_type,
+                                           dst_list_type,
+                                           expert_num=4)
+                self.assertTrue(
+                    torch.equal(result, self.glist_dict[dst_list_type]))
+
+    def test_cumsum_group_list_invalid_type_valueerror(self):
+        with self.assertRaises(ValueError) as excinfo:
+            cumsum_group_list(self.glist_dict[0], 4, 0)
+        self.assertIn("group_list_type should be in [0, 1, 2], but received",
+                      str(excinfo.exception))
+
+    def test_cumsum_group_list_unsupported_conversion_notimplementederror(
+            self):
+        for src_list_type, dst_list_type in self.unsupport_combine:
+            with self.subTest(src=src_list_type, dst=dst_list_type):
+                with self.assertRaises(NotImplementedError) as excinfo:
+                    cumsum_group_list(self.glist_dict[0], src_list_type,
+                                      dst_list_type)
+                self.assertIn("This feature is under development.",
+                              str(excinfo.exception))
+
+
+if __name__ == '__main__':
+    unittest.main(verbosity=2)