code: def _lower_triangular_mask(shape):
      """Creates a lower-triangular boolean mask over the last 2 dimensions."""
      row_index = math_ops.cumsum(
          array_ops.ones(shape=shape, dtype=dtypes.int32), axis=-2
      )
      col_index = math_ops.cumsum(
          array_ops.ones(shape=shape, dtype=dtypes.int32), axis=-1
      )
      return math_ops.greater_equal(row_index, col_index)
  
imports: from tensorflow.python.framework import dtypes
  from tensorflow.python.framework import tensor_conversion
  from tensorflow.python.framework import tensor_shape
  from tensorflow.python.keras import backend
  from tensorflow.python.keras.engine.base_layer import Layer
  from tensorflow.python.keras.utils import control_flow_util
  from tensorflow.python.ops import array_ops
  from tensorflow.python.ops import init_ops
  from tensorflow.python.ops import math_ops
  from tensorflow.python.ops import nn
label_desc: Use image node_id 1 for calling a global function with example usage: _lower_triangular_mask(shape) and returns: math_ops