code: def attention_bias_ignore_padding(memory_padding):
    
    ret = memory_padding * large_compatible_negative(memory_padding.dtype)
    return tf.expand_dims(tf.expand_dims(ret, axis=1), axis=1)
docs: Create an bias tensor to be added to attention logits.
  
    Args:
      memory_padding: a float `Tensor` with shape [batch, memory_length].
  
    Returns:
      a `Tensor` with shape [batch, 1, 1, memory_length].