code: def create_simple_graph():
      """
      Creates a simple directed graph for testing. The node ids are integers.
  
      Returns:
          A small, directed graph with 3 nodes and 2 edges in StellarDiGraph format.
      """
  
      nodes = pd.DataFrame([-1, -2, -3], index=[1, 2, 3])
      edges = pd.DataFrame(
          [(1, 2), (2, 3)], columns=["source", "target"]
      )
      return StellarDiGraph(nodes, edges)
  
file_path: stellargraph/tests/mapper/test_directed_node_generator.py
imports: import networkx
  import pandas
  from stellargraph.mapper import DirectedGraphSAGENodeGenerator
  from stellargraph.core.graph import StellarDiGraph
  from ..test_utils.graphs import weighted_tree
label_desc: Use image node_id 1 for calling a global function with example usage: create_simple_graph() and returns: StellarDiGraph