In tensorflow 1.3 seq2seq , how to extract the attention mask? -

i using contrib.seq2seq in tensorflow r1.3, how can fetch attention matrix in below apis? seems the attention matrix alignment value return of attentionwrapper.call(), how can construct call?

def single_cell():     cell = tf.contrib.rnn.basiclstmcell(rnn_size)     return tf.contrib.rnn.dropoutwrapper(cell, input_keep_prob = keep_prob)  dec_cell = tf.contrib.rnn.multirnncell([single_cell() _ in range(num_layers)])  attn_mech = tf.contrib.seq2seq.bahdanauattention(rnn_size,                                               enc_output,                                               text_length,                                               normalize=false,                                                   name='bahdanauattention')  dec_cell = tf.contrib.seq2seq.attentionwrapper(dec_cell,                                                       attn_mech,                                                       rnn_size)    initial_state = dec_cell.zero_state(batch_size, dtype=tf.float32).clone(cell_state=enc_state)    output_layer = dense(vocab_size,                      kernel_initializer = tf.truncated_normal_initializer(mean = 0.0, stddev=0.1))