code: def attention_bias_to_padding(attention_bias, cast_fn=tf.to_float):
    
    # `attention_bias` is a large negative number in padding positions and 0.0
    # elsewhere.
    return tf.squeeze(cast_fn(tf.less(attention_bias, -1)), axis=[1, 2])
docs: Inverse of attention_bias_ignore_padding().
  
    Args:
      attention_bias: a `Tensor` with shape [batch, 1, 1, memory_length], as
        returned by attention_bias_ignore_padding().
      cast_fn: function used to cast to output type.
  
    Returns:
      a Tensor with shape [batch, memory_length] with 1.0 in padding positions
      and 0.0 in non-padding positions. Type is determined by cast_fn.