'Creates indicator matrix of non-pad elements of padded batch proposals.
Args:
num_proposals: Tensor of type tf.int32 with shape [batch_size].
max_num_proposals: Maximum number of proposals per image (integer).
Returns:
A Tensor of type tf.bool with shape [batch_size, max_num_proposals].'

def _padded_batched_proposals_indicator(self, num_proposals, max_num_proposals):

    batch_size = tf.size(num_proposals)
    tiled_num_proposals = tf.tile(tf.expand_dims(num_proposals, 1), [1, max_num_proposals])
    tiled_proposal_index = tf.tile(tf.expand_dims(tf.range(max_num_proposals), 0), [batch_size, 1])
    return tf.greater(tiled_num_proposals, tiled_proposal_index)
