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Solve the following problem using Python, implementing the functions described below, one line at a time
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Description:
 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