docstr: edges added with default origin should be identified as unknown origin
docstr_tok: ["edges","added","with","default","origin","should","be","identified","as","unknown","origin"]
code: def test_add_edge_default(self):
      """edges added with default origin should be identified as unknown origin"""
  
      sm = StructureModel()
      sm.add_edge(1, 2)
  
      assert (1, 2) in sm.edges
      assert (1, 2, "unknown") in sm.edges.data("origin")
  
imports: import pytest
  from networkx.exception import NodeNotFound
  from causalnex.structure import StructureModel
label_desc: Use image node_id 1 for calling the TestStructureModelAddEdge obj's underlying member method code with example usage: obj.test_add_edge_default() without return types