docstr: Check a single node graph can't be generated
docstr_tok: ["Check","a","single","node","graph","ca","n't","be","generated"]
code: def test_num_nodes_exception(self, num_nodes):
      """Check a single node graph can't be generated"""
      with pytest.raises(
          ValueError, match="DAG must have at least 2 nodes"
      ):
          generate_structure(num_nodes, 1)
  
imports: import operator
  from typing import Hashable, Tuple, Union
  import networkx
  import numpy
  import pandas
  import pytest
  from networkx.algorithms.dag import is_directed_acyclic_graph
  from sklearn.gaussian_process.kernels import RBF
  from causalnex.structure.data_generators.core import _sample_binary_from_latent, _sample_count_from_latent, _sample_poisson, generate_structure, nonlinear_sem_generator, sem_generator
  from causalnex.structure.structuremodel import StructureModel
label_desc: Use image node_id 6 for calling the TestGenerateStructure obj's underlying member method code with example usage: obj.test_num_nodes_exception(num_nodes) without return types