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_edges_from_default(self):
      """edges added with default origin should be identified as unknown origin"""
  
      sm = StructureModel()
      edges = [(1, 2), (2, 3)]
      sm.add_edges_from(edges)
  
      assert all(edge in sm.edges for edge in edges)
      assert all(
          (u, v, "unknown") in sm.edges.data("origin") for u, v in edges
      )
  
imports: import pytest
  from networkx.exception import NodeNotFound
  from causalnex.structure import StructureModel
label_desc: Use image node_id 1 for calling the TestStructureModelAddEdgesFrom obj's underlying member method code with example usage: obj.test_add_edges_from_default() without return types