docstr: it should be possible to add an edge with custom attributes
docstr_tok: ["it","should","be","possible","to","add","an","edge","with","custom","attributes"]
code: def test_add_edge_custom_attr(self):
      """it should be possible to add an edge with custom attributes"""
  
      sm = StructureModel()
      sm.add_edge(1, 2, x="Y")
  
      assert (1, 2) in sm.edges
      assert (1, 2, "Y") in sm.edges.data("x")
  
imports: import pytest
  from networkx.exception import NodeNotFound
  from causalnex.structure import StructureModel
label_desc: Use image node_id 6 for calling the TestStructureModelAddEdge obj's underlying member method code with example usage: obj.test_add_edge_custom_attr() without return types