code: def get_feedforward_adj_mat(num_layers):
      """Returns an adjacency matrix for a feed forward network."""
      ret = dok_matrix((num_layers, num_layers))
      for i in range(num_layers - 1):
          ret[i, i + 1] = 1
      return ret
  
imports: import numpy
  from scipy.sparse import dok_matrix
  from .neural_network import ConvNeuralNetwork, MultiLayerPerceptron, get_cnn_layer_labels, get_mlp_layer_labels, is_a_conv_layer_label
  from .nn_modifiers import NNModifier
  from ..utils.general_utils import get_dok_mat_with_set_coords
label_desc: Use image node_id 1 for calling a global function with example usage: get_feedforward_adj_mat(num_layers) and returns: ret