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].

def attention_bias_ignore_padding(memory_padding):
  """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].
  """
  ret = memory_padding * large_compatible_negative(memory_padding.dtype)
  return tf.expand_dims(tf.expand_dims(ret, axis=1), axis=1)