import tensorflow as tf

        for layer in range(num_layers):
            input_size_ = input_size if layer == 0 else 2 * num_units
            gru_fw = tf.contrib.cudnn_rnn.CudnnGRU(1, num_units)
            gru_bw = tf.contrib.cudnn_rnn.CudnnGRU(1, num_units)
            init_fw = tf.tile(tf.Variable(
                tf.zeros([1, 1, num_units])), [1, batch_size, 1])
