support one node communication group

mindspore/communication/_comm_helper.py

@@ -334,8 +334,8 @@ def _create_group_helper(group, rank_ids, backend):
         if not isinstance(rank_ids, list):
             raise TypeError("Rank_ids {} should be list".format(rank_ids))
         rank_size = len(rank_ids)
-        if rank_size < 2:
-            raise ValueError("Rank_ids size {} should be large than 1".format(rank_size))
+        if rank_size < 1:
+            raise ValueError("Rank_ids size {} should be large than 0".format(rank_size))
         if len(rank_ids) - len(list(set(rank_ids))) > 0:
             raise ValueError("List rank_ids in Group {} has duplicate data!".format(group))
         hccl.create_group(group, rank_size, rank_ids)

tests/ut/python/communication/test_management_api.py

@@ -99,7 +99,7 @@ def test_raise_error_funcs():
     assert has_raise_error(create_backend, 'nccl') is False
     assert has_raise_error(get_group_size_int, 123) is True
     assert has_raise_error(create_group0, (0,1)) is True
-    assert has_raise_error(create_group1, [0]) is True
+    assert has_raise_error(create_group1, [0]) is False
     assert has_raise_error(create_group2, [0,0,1]) is True
     assert has_raise_error(create_group3, [0,1]) is True
     assert has_raise_error(create_group4, [0,1]) is False