code: def get_self_attention_bias(x):
      """Creates masked self attention bias.
  
      Args:
        x: A tensor of shape [batch, length, depth]
  
      Returns:
        self_attention_bias: A tensor of shape [length, length, 1]
      """
  
      x_shape = common_layers.shape_list(x)
      self_attention_bias = (
          common_attention.attention_bias_lower_triangle(x_shape[1])
      )
      return self_attention_bias
  
imports: from __future__ import absolute_import
  from __future__ import division
  from __future__ import print_function
  import numpy
  from six.moves import range
  from tensor2tensor.layers import common_attention
  from tensor2tensor.layers import common_layers
  from tensor2tensor.utils import expert_utils
  import tensorflow.compat.v1
  from tensorflow.compat.v1 import estimator
label_desc: Use image node_id 13 for calling a global function with example usage: get_self_attention_bias(x) and returns: self_attention_bias