<p>This type of repetition is called a <strong>"text degeneration"</strong>.</p>

<p>There is a great paper from 2019 which analyse this phenomenon: <strong><a href="https://arxiv.org/abs/1904.09751" rel="noreferrer">The Curious Case of Neural Text Degeneration</a></strong> by <strong>Ari Holtzman</strong> et al. from the Allen Institute for Artificial Intelligence.</p>

<p>The repetition may come from the type of text search (text sampling) on the decoder site. Many people implement this just by the most probable next world proposed by the model (argmax on the softmax on the last layer) or by so called beam search. In fact the beam search is the industry standard for today.</p>

<p>This is the example of Beam search from the article:</p>

<p><strong>Continuation (BeamSearch, b=10):</strong></p>

<p><em>"The unicorns were able to communicate with each other, they said unicorns. a statement that the unicorns. Professor of the Department of Los Angeles, the most important place the world to be recognition <strong>of the world to be a of the world to be a of the world to be a of the world to be a of the world to be a of the world to be a of the world to be a of the world to be a of the…</em></strong></p>

<p>As you can see there is a great amount of repetition.</p>

<p>According to the paper this curious case may be explained by the fact that each repeated sequence of words have higher probability than the sequence without the next repetition:
<a href="https://i.stack.imgur.com/I0iUG.png" rel="noreferrer"><img src="https://i.stack.imgur.com/I0iUG.png" alt="enter image description here"></a></p>

<p>The article propose some workarounds with words sampling by the decoder. It definitely requires more study, but this is the best explanation we have today.</p>

<p>The other is that your model need still more training. In many cases I faced a similar behaviour when I had big training set and model still couldn't generalise well over whole diversity of the data. To test this hypothesis - try to train on smaller dataset and see if it generalise (produce meaningful results). </p>

<p>But even if your model generalise well enough, that doesn't mean you won't ever face the repetition pattern. Unless you change the sampling patter of the decoder, it is a common scenario.</p>
What to do when Seq2Seq network repeats words over and over in output?
<p>So, I've been working on a project for a while, we have <em>very</em> little data, I know it would become much better if we were able to put together a much much larger dataset. That aside, my issue at the moment is when I have a sentence input, my outputs look like this right now:</p>

<blockquote>
  <p>contactid contactid contactid contactid</p>
</blockquote>

<p>A single word is focused on and repeated over and over again. What can I do to overcome this hurdle?</p>

<p>Things I've tried:</p>

<ol>
<li>Double checked I was appending start/stop tokens and make sure the tokens were properly placed in the top of their vocab files, I am sharing vocab.</li>
<li>I found something saying it could be due to poor word embeddings. To that end I checked with tensorboard and sure enough PCA showed a very dense cluster of points. Seeing that I grabbed Facebook's public pre trained word vectors and loaded them in as the embedding. Trained again and this time tensorboard PCA showed a much better picture.</li>
<li>Switched my training scheduler from basic to SampledScheduling to occasionally replace a training output with the ground truth.</li>
<li>Switched my decoder to use the beam search decoder I figured this may give more robust responses if the word choices were close together in the intermediary feature space.</li>
</ol>

<p>For certain my perplexity is steadily decreasing.</p>

<p>Here is my dataset preperation code:</p>

<pre><code>class ModelInputs(object):
"""Factory to construct various input hooks and functions depending on mode """

def __init__(
    self, vocab_files, batch_size,
    share_vocab=True, src_eos_id=1, tgt_eos_id=2
):
    self.batch_size = batch_size
    self.vocab_files = vocab_files
    self.share_vocab = share_vocab
    self.src_eos_id = src_eos_id
    self.tgt_eos_id = tgt_eos_id

def get_inputs(self, file_path, num_infer=None, mode=tf.estimator.ModeKeys.TRAIN):
    self.mode = mode
    if self.mode == tf.estimator.ModeKeys.TRAIN:
        return self._training_input_hook(file_path)
    if self.mode == tf.estimator.ModeKeys.EVAL:
        return self._validation_input_hook(file_path)
    if self.mode == tf.estimator.ModeKeys.PREDICT:
        if num_infer is None:
            raise ValueError('If performing inference must supply number of predictions to be made.')
        return self._infer_input_hook(file_path, num_infer)

def _prepare_data(self, dataset, out=False):
    prep_set = dataset.map(lambda string: tf.string_split([string]).values)
    prep_set = prep_set.map(lambda words: (words, tf.size(words)))
    if out == True:
        return prep_set.map(lambda words, size: (self.vocab_tables[1].lookup(words), size))
    return prep_set.map(lambda words, size: (self.vocab_tables[0].lookup(words), size))

def _batch_data(self, dataset, src_eos_id, tgt_eos_id):
    batched_set = dataset.padded_batch(
            self.batch_size,
            padded_shapes=((tf.TensorShape([None]), tf.TensorShape([])), (tf.TensorShape([None]), tf.TensorShape([]))),
            padding_values=((src_eos_id, 0), (tgt_eos_id, 0))
    )
    return batched_set

def _batch_infer_data(self, dataset, src_eos_id):
    batched_set = dataset.padded_batch(
        self.batch_size,
        padded_shapes=(tf.TensorShape([None]), tf.TensorShape([])),
        padding_values=(src_eos_id, 0)
    )
    return batched_set

def _create_vocab_tables(self, vocab_files, share_vocab=False):
    if vocab_files[1] is None and share_vocab == False:
        raise ValueError('If share_vocab is set to false must provide target vocab. (src_vocab_file, \
                target_vocab_file)')

    src_vocab_table = lookup_ops.index_table_from_file(
        vocab_files[0],
        default_value=UNK_ID
    )

    if share_vocab:
        tgt_vocab_table = src_vocab_table
    else:
        tgt_vocab_table = lookup_ops.index_table_from_file(
            vocab_files[1],
            default_value=UNK_ID
        )

    return src_vocab_table, tgt_vocab_table

def _prepare_iterator_hook(self, hook, scope_name, iterator, file_path, name_placeholder):
    if self.mode == tf.estimator.ModeKeys.TRAIN or self.mode == tf.estimator.ModeKeys.EVAL:
        feed_dict = {
                name_placeholder[0]: file_path[0],
                name_placeholder[1]: file_path[1]
        }
    else:
        feed_dict = {name_placeholder: file_path}

    with tf.name_scope(scope_name):
        hook.iterator_initializer_func = \
                lambda sess: sess.run(
                    iterator.initializer,
                    feed_dict=feed_dict,
                )

def _set_up_train_or_eval(self, scope_name, file_path):
    hook = IteratorInitializerHook()
    def input_fn():
        with tf.name_scope(scope_name):
            with tf.name_scope('sentence_markers'):
                src_eos_id = tf.constant(self.src_eos_id, dtype=tf.int64)
                tgt_eos_id = tf.constant(self.tgt_eos_id, dtype=tf.int64)
            self.vocab_tables = self._create_vocab_tables(self.vocab_files, self.share_vocab)
            in_file = tf.placeholder(tf.string, shape=())
            in_dataset = self._prepare_data(tf.contrib.data.TextLineDataset(in_file).repeat(None))
            out_file = tf.placeholder(tf.string, shape=())
            out_dataset = self._prepare_data(tf.contrib.data.TextLineDataset(out_file).repeat(None))
            dataset = tf.contrib.data.Dataset.zip((in_dataset, out_dataset))
            dataset = self._batch_data(dataset, src_eos_id, tgt_eos_id)
            iterator = dataset.make_initializable_iterator()
            next_example, next_label = iterator.get_next()
            self._prepare_iterator_hook(hook, scope_name, iterator, file_path, (in_file, out_file))
            return next_example, next_label

    return (input_fn, hook)

def _training_input_hook(self, file_path):
    input_fn, hook = self._set_up_train_or_eval('train_inputs', file_path)

    return (input_fn, hook)

def _validation_input_hook(self, file_path):
    input_fn, hook = self._set_up_train_or_eval('eval_inputs', file_path)

    return (input_fn, hook)

def _infer_input_hook(self, file_path, num_infer):
    hook = IteratorInitializerHook()

    def input_fn():
        with tf.name_scope('infer_inputs'):
            with tf.name_scope('sentence_markers'):
                src_eos_id = tf.constant(self.src_eos_id, dtype=tf.int64)
            self.vocab_tables = self._create_vocab_tables(self.vocab_files, self.share_vocab)
            infer_file = tf.placeholder(tf.string, shape=())
            dataset = tf.contrib.data.TextLineDataset(infer_file)
            dataset = self._prepare_data(dataset)
            dataset = self._batch_infer_data(dataset, src_eos_id)
            iterator = dataset.make_initializable_iterator()
            next_example, seq_len = iterator.get_next()
            self._prepare_iterator_hook(hook, 'infer_inputs', iterator, file_path, infer_file)
            return ((next_example, seq_len), None)

    return (input_fn, hook)
</code></pre>

<p>And here is my model:</p>

<pre><code>class Seq2Seq():

def __init__(
    self, batch_size, inputs,
    outputs, inp_vocab_size, tgt_vocab_size,
    embed_dim, mode, time_major=False,
    enc_embedding=None, dec_embedding=None, average_across_batch=True,
    average_across_timesteps=True, vocab_path=None, embedding_path='./data_files/wiki.simple.vec'
):
    embed_np = self._get_embedding(embedding_path)
    if not enc_embedding:
        self.enc_embedding = tf.contrib.layers.embed_sequence(
            inputs,
            inp_vocab_size,
            embed_dim,
            trainable=True,
            scope='embed',
            initializer=tf.constant_initializer(value=embed_np, dtype=tf.float32)
        )
    else:
        self.enc_embedding = enc_embedding
    if mode == tf.estimator.ModeKeys.TRAIN or mode == tf.estimator.ModeKeys.EVAL:
        if not dec_embedding:
            embed_outputs = tf.contrib.layers.embed_sequence(
                outputs,
                tgt_vocab_size,
                embed_dim,
                trainable=True,
                scope='embed',
                reuse=True
            )
            with tf.variable_scope('embed', reuse=True):
                dec_embedding = tf.get_variable('embeddings')
            self.embed_outputs = embed_outputs
            self.dec_embedding = dec_embedding

        else:
            self.dec_embedding = dec_embedding
    else:
        with tf.variable_scope('embed', reuse=True):
            self.dec_embedding = tf.get_variable('embeddings')

    if mode == tf.estimator.ModeKeys.PREDICT and vocab_path is None:
        raise ValueError('If mode is predict, must supply vocab_path')
    self.vocab_path = vocab_path
    self.inp_vocab_size = inp_vocab_size
    self.tgt_vocab_size = tgt_vocab_size
    self.average_across_batch = average_across_batch
    self.average_across_timesteps = average_across_timesteps
    self.time_major = time_major
    self.batch_size = batch_size
    self.mode = mode

def _get_embedding(self, embedding_path):
    model = KeyedVectors.load_word2vec_format(embedding_path)
    vocab = model.vocab
    vocab_len = len(vocab)
    return np.array([model.word_vec(k) for k in vocab.keys()])

def _get_lstm(self, num_units):
    return tf.nn.rnn_cell.BasicLSTMCell(num_units)

def encode(self, num_units, num_layers, seq_len, cell_fw=None, cell_bw=None):
    if cell_fw and cell_bw:
        fw_cell = cell_fw
        bw_cell = cell_bw
    else:
        fw_cell = self._get_lstm(num_units)
        bw_cell = self._get_lstm(num_units)
    encoder_outputs, bi_encoder_state = tf.nn.bidirectional_dynamic_rnn(
        fw_cell,
        bw_cell,
        self.enc_embedding,
        sequence_length=seq_len,
        time_major=self.time_major,
        dtype=tf.float32
    )
    c_state = tf.concat([bi_encoder_state[0].c, bi_encoder_state[1].c], axis=1)
    h_state = tf.concat([bi_encoder_state[0].h, bi_encoder_state[1].h], axis=1)
    encoder_state = tf.contrib.rnn.LSTMStateTuple(c=c_state, h=h_state)
    return tf.concat(encoder_outputs, -1), encoder_state

def _train_decoder(self, decoder_cell, out_seq_len, encoder_state, helper):
    if not helper:
        helper = tf.contrib.seq2seq.ScheduledEmbeddingTrainingHelper(
            self.embed_outputs,
            out_seq_len,
            self.dec_embedding,
            0.3,
        )
        # helper = tf.contrib.seq2seq.TrainingHelper(
        #     self.dec_embedding,
        #     out_seq_len,
        # )
    projection_layer = layers_core.Dense(self.tgt_vocab_size, use_bias=False)
    decoder = tf.contrib.seq2seq.BasicDecoder(
        decoder_cell,
        helper,
        encoder_state,
        output_layer=projection_layer
    )
    return decoder

def _predict_decoder(self, cell, encoder_state, beam_width, length_penalty_weight):
    tiled_encoder_state = tf.contrib.seq2seq.tile_batch(
        encoder_state, multiplier=beam_width
    )
    with tf.name_scope('sentence_markers'):
        sos_id = tf.constant(1, dtype=tf.int32)
        eos_id = tf.constant(2, dtype=tf.int32)
    start_tokens = tf.fill([self.batch_size], sos_id)
    end_token = eos_id
    projection_layer = layers_core.Dense(self.tgt_vocab_size, use_bias=False)
    emb = tf.squeeze(self.dec_embedding)
    decoder = tf.contrib.seq2seq.BeamSearchDecoder(
        cell=cell,
        embedding=self.dec_embedding,
        start_tokens=start_tokens,
        end_token=end_token,
        initial_state=tiled_encoder_state,
        beam_width=beam_width,
        output_layer=projection_layer,
        length_penalty_weight=length_penalty_weight
    )
    return decoder

def decode(
    self, num_units, out_seq_len,
    encoder_state, cell=None, helper=None,
    beam_width=None, length_penalty_weight=None
):
    with tf.name_scope('Decode'):
        if cell:
            decoder_cell = cell
        else:
            decoder_cell = tf.nn.rnn_cell.BasicLSTMCell(2*num_units)
        if self.mode != estimator.ModeKeys.PREDICT:
            decoder = self._train_decoder(decoder_cell, out_seq_len, encoder_state, helper)
        else:
            decoder = self._predict_decoder(decoder_cell, encoder_state, beam_width, length_penalty_weight)
        outputs = tf.contrib.seq2seq.dynamic_decode(
            decoder,
            maximum_iterations=20,
            swap_memory=True,
        )
        outputs = outputs[0]
        if self.mode != estimator.ModeKeys.PREDICT:
            return outputs.rnn_output, outputs.sample_id
        else:
            return outputs.beam_search_decoder_output, outputs.predicted_ids

def prepare_predict(self, sample_id):
    rev_table = lookup_ops.index_to_string_table_from_file(
        self.vocab_path, default_value=UNK)
    predictions = rev_table.lookup(tf.to_int64(sample_id))
    return tf.estimator.EstimatorSpec(
        predictions=predictions,
        mode=tf.estimator.ModeKeys.PREDICT
    )

def prepare_train_eval(
    self, t_out,
    out_seq_len, labels, lr,
    train_op=None, loss=None
):
    if not loss:
        weights = tf.sequence_mask(
            out_seq_len,
            dtype=t_out.dtype
        )
        loss = tf.contrib.seq2seq.sequence_loss(
            t_out,
            labels,
            weights,
            average_across_batch=self.average_across_batch,
        )

    if not train_op:
        train_op = tf.contrib.layers.optimize_loss(
            loss,
            tf.train.get_global_step(),
            optimizer='SGD',
            learning_rate=lr,
            summaries=['loss', 'learning_rate']
        )

    return tf.estimator.EstimatorSpec(
        mode=self.mode,
        loss=loss,
        train_op=train_op,
    )
</code></pre>