tf.nn.bidirectional_dynamic_rnn在Tensorflow中返回单个值

Question

tf.nn.bidirectional_dynamic_rnn通常按以下方式使用。

 encoder_outputs, encoder_state = tf.nn.bidirectional_dynamic_rnn(cell_fw=fw_cell, cell_bw=bw_cell, 
                                                                  inputs=inputs, sequence_length=sequence_size,
                                                                  dtype=tf.float64)

但是，当我看到以下代码时，我不确定_中返回什么值。

output = tf.nn.bidirectional_dynamic_rnn(fw_cell, bw_cell, dtype=tf.float32,
                                          inputs = X, sequence_length = seq_len)
_, ((_, output_fw), (_, output_bw)) = output

您能告诉我_返回什么吗？