added beam_search generation for tf 2.0

src/transformers/modeling_tf_utils.py

@@ -142,7 +142,7 @@ class TFPreTrainedModel(tf.keras.Model, TFModelUtilsMixin):
         # # initialize all new embeddings (in particular added tokens)
         # self._init_weights(new_embeddings)
 
-        # # Copy word embeddings from the previous weights
+        # # Copy token embeddings from the previous weights
         # num_tokens_to_copy = min(old_num_tokens, new_num_tokens)
         # new_embeddings.weight.data[:num_tokens_to_copy, :] = old_embeddings.weight.data[:num_tokens_to_copy, :]
 
@@ -557,6 +557,19 @@ class TFPreTrainedModel(tf.keras.Model, TFModelUtilsMixin):
         else:
             assert len(shape_list(input_ids)) == 2, "Input prompt should be of shape (batch_size, sequence length)."
 
+        if do_sample is False:
+            if num_beams == 1:
+                # no_beam_search greedy generation conditions
+                assert (
+                    num_return_sequences == 1
+                ), "Greedy decoding will always produce the same output for num_beams == 1 and num_return_sequences > 1. Please set num_return_sequences = 1"
+
+            else:
+                # beam_search greedy generation conditions
+                assert (
+                    num_beams >= num_return_sequences
+                ), "Greedy beam search decoding cannot return more sequences than it has beams. Please set num_beams >= num_return_sequences"
+
         if pad_token_id is None and eos_token_ids is not None:
             logger.warning(
                 "Setting `pad_token_id` to {} (first `eos_token_id`) to generate sequence".format(eos_token_ids[0])
@@ -567,7 +580,7 @@ class TFPreTrainedModel(tf.keras.Model, TFModelUtilsMixin):
         cur_len = shape_list(input_ids)[1]
         vocab_size = self.config.vocab_size
 
-        if num_return_sequences != 1:
+        if num_return_sequences != 1 and do_sample:
             # Expand input to num return sequences
             input_ids = tf.broadcast_to(tf.expand_dims(input_ids, 1), (batch_size, num_return_sequences, cur_len))
             effective_batch_size = batch_size * num_return_sequences
@@ -588,6 +601,7 @@ class TFPreTrainedModel(tf.keras.Model, TFModelUtilsMixin):
                 pad_token_id,
                 eos_token_ids,
                 effective_batch_size,
+                num_return_sequences,
                 length_penalty,
                 num_beams,
                 vocab_size,
@@ -627,19 +641,7 @@ class TFPreTrainedModel(tf.keras.Model, TFModelUtilsMixin):
             All returned sequence are generated independantly.
         """
 
-        def _create_next_token_logits_penalties(input_ids, logits):
-            # create logit penalties for already seen input_ids
-            token_penalties = np.ones(shape_list(logits))
-            prev_input_ids = [np.unique(input_id) for input_id in input_ids.numpy()]
-            for i, prev_input_id in enumerate(prev_input_ids):
-                logit_penalized = logits[i].numpy()[prev_input_id]
-                # if previous logit score is < 0 then multiply repetition penalty else divide
-                logit_penalized[logit_penalized < 0] = repetition_penalty
-                logit_penalized[logit_penalized > 0] = 1 / repetition_penalty
-                np.put(token_penalties[i], prev_input_id, logit_penalized)
-            return tf.convert_to_tensor(token_penalties, dtype=tf.float32)
-
-        # current position / max lengths / length of generated sentences / unfinished sentences
+        # length of generated sentences / unfinished sentences
         unfinished_sents = tf.ones_like(input_ids[:, 0])
         sent_lengths = tf.ones_like(input_ids[:, 0]) * max_length
 
@@ -656,7 +658,7 @@ class TFPreTrainedModel(tf.keras.Model, TFModelUtilsMixin):
 
             # repetition penalty from CTRL paper (https://arxiv.org/abs/1909.05858)
             if repetition_penalty != 1.0:
-                next_token_logits_penalties = _create_next_token_logits_penalties(input_ids, next_token_logits)
+                next_token_logits_penalties = _create_next_token_logits_penalties(input_ids, next_token_logits, repetition_penalty)
                 next_token_logits = tf.math.multiply(next_token_logits, next_token_logits_penalties)
 
             if do_sample:
@@ -738,11 +740,228 @@ class TFPreTrainedModel(tf.keras.Model, TFModelUtilsMixin):
         pad_token_id,
         eos_token_ids,
         batch_size,
+        num_return_sequences,
         length_penalty,
         num_beams,
         vocab_size,
     ):
-        pass
+        """ Generate sequences for each example with beam search.
+        """
+
+        # Expand input to num beams
+        input_ids = tf.broadcast_to(tf.expand_dims(input_ids, 1), (batch_size, num_beams, cur_len))
+        input_ids = tf.reshape(input_ids, (batch_size * num_beams, cur_len))  # (batch_size * num_beams, cur_len)
+
+        # generated hypotheses
+        generated_hyps = [
+            BeamHypotheses(num_beams, max_length, length_penalty, early_stopping=False) for _ in range(batch_size)
+        ]
+
+        # scores for each sentence in the beam
+        beam_scores_begin = tf.zeros((batch_size, 1), dtype=tf.float32)
+        beam_scores_end = tf.zeros((batch_size, num_beams - 1), dtype=tf.float32) * 1e-9
+        beam_scores = tf.reshape(tf.concat([beam_scores_begin, beam_scores_end], -1), (batch_size * num_beams,))
+
+        # cache compute states
+        past = None
+
+        # done sentences
+        done = [False for _ in range(batch_size)]
+
+        while cur_len < max_length:
+            model_inputs = self.prepare_inputs_for_generation(input_ids, past=past)
+            outputs = self(**model_inputs)  # (batch_size * num_beams, cur_len, vocab_size)
+            next_token_logits = outputs[0][:, -1, :]  # (batch_size * num_beams, vocab_size)
+
+            # if model has past, then set the past variable to speed up decoding
+            if self._do_output_past(outputs):
+                past = outputs[1]
+
+            # repetition penalty (from CTRL paper https://arxiv.org/abs/1909.05858)
+            if repetition_penalty != 1.0:
+                next_token_logits_penalties = _create_next_token_logits_penalties(input_ids, next_token_logits, repetition_penalty)
+                next_token_logits = tf.math.multiply(next_token_logits, next_token_logits_penalties)
+
+            if do_sample:
+                # Temperature (higher temperature => more likely to sample low probability tokens)
+                if temperature != 1.0:
+                    next_token_logits = next_token_logits / temperature
+                # Top-p/top-k filtering
+                next_token_logits = tf_top_k_top_p_filtering(
+                    next_token_logits, top_k=top_k, top_p=top_p, min_tokens_to_keep=2
+                )  # (batch_size * num_beams, vocab_size)
+                # Sample 2 next tokens for each beam (so we have some spare tokens and match output of greedy beam search)
+                next_tokens = tf.random.categorical(next_token_logits, dtype=tf.int32, num_samples=2)  # (batch_size * num_beams, vocab_size)
+                # Compute next scores
+                scores = tf.nn.log_softmax(next_token_logits, axis=-1)  # (batch_size * num_beams, vocab_size)
+                _scores = tf.gather(scores, next_tokens, batch_dims=1)  # (batch_size * num_beams, 2)
+                next_scores = _scores + tf.broadcast_to(beam_scores[:, None], (batch_size * num_beams, 2))  # (batch_size * num_beams, 2)
+                # Match shape of greedy beam search
+                next_tokens = tf.reshape(next_tokens, (batch_size, 2 * num_beams))  # (batch_size, 2 * num_beams)
+                next_scores = tf.reshape(next_scores, (batch_size, 2 * num_beams))  # (batch_size, 2 * num_beams)
+            else:
+                # do greedy beam search
+                scores = tf.nn.log_softmax(next_token_logits, axis=-1)  # (batch_size * num_beams, vocab_size)
+                assert shape_list(scores) == [batch_size * num_beams, vocab_size]
+                # Add the log prob of the new beams to the log prob of the beginning of the sequence (sum of logs == log of the product)
+                next_scores = scores + tf.broadcast_to(beam_scores[:, None], (batch_size * num_beams, vocab_size))  # (batch_size * num_beams, vocab_size)
+
+                # re-organize to group the beam together (we are keeping top hypothesis accross beams)
+                next_scores = tf.reshape(next_scores, (batch_size, num_beams * vocab_size))  # (batch_size, num_beams * vocab_size)
+                next_scores, next_tokens = tf.math.top_k(next_scores, 2 * num_beams, sorted=True)
+
+            assert shape_list(next_scores) == shape_list(next_tokens) == [batch_size, 2 * num_beams]
+
+            # next batch beam content
+            # list of (batch_size * num_beams) tuple(next hypothesis score, next token, current position in the batch)
+            next_batch_beam = []
+
+            # for each sentence
+            for batch_idx in range(batch_size):
+
+                # if we are done with this sentence
+                done[batch_idx] = done[batch_idx] or generated_hyps[batch_idx].is_done(
+                    tf.reduce_max(next_scores[batch_idx]).numpy()
+                )
+                if done[batch_idx]:
+                    assert (
+                        len(generated_hyps[batch_idx]) >= num_beams
+                    ), "Batch can only be done if at least {} beams have been generated".format(num_beams)
+                    assert (
+                        eos_token_ids is not None and pad_token_id is not None
+                    ), "generated beams >= num_beams -> eos_token_id and pad_token have to be defined"
+                    next_batch_beam.extend([(0, pad_token_id, 0)] * num_beams)  # pad the batch
+                    continue
+
+                # next sentence beam content
+                next_sent_beam = []
+
+                # next tokens for this sentence
+                for idx, score in zip(next_tokens[batch_idx], next_scores[batch_idx]):
+
+                    # get beam and token IDs
+                    beam_id = idx // vocab_size
+                    token_id = idx % vocab_size
+
+                    # add to generated hypotheses if end of sentence or last iteration
+                    if eos_token_ids is not None and token_id.numpy() in eos_token_ids:
+                        generated_hyps[batch_idx].add(
+                            tf.identity(input_ids[batch_idx * num_beams + beam_id, :cur_len]), score.numpy()
+                        )
+                    else:
+                        # add next predicted token if it is not eos_token
+                        next_sent_beam.append((score, token_id, batch_idx * num_beams + beam_id))
+
+                    # the beam for next step is full
+                    if len(next_sent_beam) == num_beams:
+                        break
+
+                # update next beam content
+                assert len(next_sent_beam) == num_beams, "Beam should always be full"
+                next_batch_beam.extend(next_sent_beam)
+                assert len(next_batch_beam) == num_beams * (batch_idx + 1)
+
+            # sanity check / prepare next batch
+            assert len(next_batch_beam) == batch_size * num_beams
+            beam_scores = tf.convert_to_tensor([x[0] for x in next_batch_beam], dtype=tf.float32)
+            beam_tokens = tf.convert_to_tensor([x[1] for x in next_batch_beam], dtype=tf.int32)
+            beam_idx = tf.convert_to_tensor([x[2] for x in next_batch_beam], dtype=tf.int32)
+
+            # re-order batch
+            input_ids = tf.stack([tf.identity(input_ids[x, :]) for x in beam_idx])
+            input_ids = tf.concat([input_ids, tf.expand_dims(beam_tokens, 1)], axis=-1)
+
+            # re-order internal states
+            if past:
+                past = self._reorder_cache(past, beam_idx)
+
+            # update current length
+            cur_len = cur_len + 1
+
+            # stop when we are done with each sentence
+            if all(done):
+                break
+
+        for batch_idx in range(batch_size):
+            # Add all open beam hypothesis to generated_hyps
+            if not done[batch_idx]:
+                for idx, score in zip(next_tokens[batch_idx], next_scores[batch_idx]):
+
+                    # get beam and token IDs
+                    beam_id = idx // vocab_size
+                    token_id = idx % vocab_size
+                    generated_hyps[batch_idx].add(
+                        tf.identity(input_ids[batch_idx * num_beams + beam_id, :cur_len]), score.numpy()
+                    )
+
+        # depending on whether greedy generation is wanted or not define different output_batch_size and output_num_return_sequences_per_batch
+        output_batch_size = batch_size if do_sample else batch_size * num_return_sequences
+        output_num_return_sequences_per_batch = 1 if do_sample else num_return_sequences
+
+        # select the best hypotheses
+        sent_lengths_list = []
+        best = []
+
+        # retrieve best hypotheses
+        for i, hypotheses in enumerate(generated_hyps):
+            sorted_hyps = sorted(hypotheses.beams, key=lambda x: x[0])
+            for j in range(output_num_return_sequences_per_batch):
+                best_hyp = sorted_hyps.pop()[1]
+                sent_lengths_list.append(len(best_hyp))
+                best.append(best_hyp)
+        assert output_batch_size == len(best), "Output batch size {} must match output beam hypotheses {}".format(output_batch_size, len(best))
+
+        sent_lengths = tf.convert_to_tensor(sent_lengths_list, dtype=tf.int32)
+
+        # shorter batches are filled with pad_token
+        if tf.reduce_min(sent_lengths).numpy() != tf.reduce_max(sent_lengths).numpy():
+            assert pad_token_id is not None, "`Pad_token_id` has to be defined"
+            sent_max_len = min(tf.reduce_max(sent_lengths).numpy() + 1, max_length)
+            decoded_list = []
+
+            # fill with hypothesis and eos_token_id if necessary
+            for i, hypo in enumerate(best):
+                padding = tf.ones((sent_max_len - shape_list(hypo)[0],), dtype=tf.int32) * pad_token_id
+                decoded_hypo = tf.concat([hypo, padding], axis=0)
+
+                if sent_lengths[i] < max_length:
+                    decoded_hypo = tf.where(tf.range(max_length) == sent_lengths[i], eos_token_ids[0] * tf.ones((sent_max_len,), dtype=tf.int32), decoded_hypo)
+                decoded_list.append(decoded_hypo)
+            decoded = tf.stack(decoded_list)
+        else:
+            # none of the hypotheses have an eos_token
+            assert (len(hypo) == max_length for hypo in best)
+            decoded = tf.stack(best)
+
+        return decoded
+
+    @staticmethod
+    def _reorder_cache(past, beam_idx):
+        reordered_past = []
+        for layer_past in past:
+            # get the correct batch idx from layer past batch dim
+            # batch dim of `past` and `mems` is at 2nd position
+            reordered_layer_past = [tf.identity(tf.expand_dims(layer_past[i], 0)) for i in beam_idx]
+            # TODO: check whether it is an error that TF past.shape != Torch past.shape
+            reordered_layer_past = tf.concat(reordered_layer_past, axis=0)
+            # check that shape matches
+            assert shape_list(reordered_layer_past) == shape_list(layer_past)
+            reordered_past.append(reordered_layer_past)
+        past = tuple(reordered_past)
+        return past
+
+
+def _create_next_token_logits_penalties(input_ids, logits, repetition_penalty):
+    # create logit penalties for already seen input_ids
+    token_penalties = np.ones(shape_list(logits))
+    prev_input_ids = [np.unique(input_id) for input_id in input_ids.numpy()]
+    for i, prev_input_id in enumerate(prev_input_ids):
+        logit_penalized = logits[i].numpy()[prev_input_id]
+        # if previous logit score is < 0 then multiply repetition penalty else divide
+        logit_penalized[logit_penalized < 0] = repetition_penalty
+        logit_penalized[logit_penalized > 0] = 1 / repetition_penalty
+        np.put(token_penalties[i], prev_input_id, logit_penalized)
+    return tf.convert_to_tensor(token_penalties, dtype=tf.float32)
 
 
 def tf_top_k_top_p_filtering(logits, top_k=0, top_p=1.0, filter_value=-float("Inf"), min_tokens_to_keep=1):
@@ -811,6 +1030,56 @@ def set_tensor_by_indices_to_value(tensor, indices, value):
     return tf.where(indices, value_tensor, tensor)
 
 
+class BeamHypotheses(object):
+    def __init__(self, num_beams, max_length, length_penalty, early_stopping):
+        """
+        Initialize n-best list of hypotheses.
+        """
+        self.max_length = max_length - 1  # ignoring bos_token
+        self.length_penalty = length_penalty
+        self.early_stopping = early_stopping
+        self.num_beams = num_beams
+        self.beams = []
+        self.worst_score = 1e9
+
+    def __len__(self):
+        """
+        Number of hypotheses in the list.
+        """
+        return len(self.beams)
+
+    def add(self, hyp, sum_logprobs):
+        """
+        Add a new hypothesis to the list.
+        """
+        score = sum_logprobs / len(hyp) ** self.length_penalty
+        if len(self) < self.num_beams or score > self.worst_score:
+            self.beams.append((score, hyp))
+            if len(self) > self.num_beams:
+                sorted_scores = sorted([(s, idx) for idx, (s, _) in enumerate(self.beams)])
+                del self.beams[sorted_scores[0][1]]
+                self.worst_score = sorted_scores[1][0]
+            else:
+                self.worst_score = min(score, self.worst_score)
+
+    def is_done(self, best_sum_logprobs, cur_len=None):
+        """
+        If there are enough hypotheses and that none of the hypotheses being generated
+        can become better than the worst one in the heap, then we are done with this sentence.
+        """
+
+        if len(self) < self.num_beams:
+            return False
+        elif self.early_stopping:
+            return True
+        else:
+            if cur_len is None:
+                cur_len = self.max_length
+            cur_score = best_sum_logprobs / cur_len ** self.length_penalty
+            ret = self.worst_score >= cur_score
+            return ret
+
+
 class TFConv1D(tf.keras.layers.Layer):
     def __init__(self, nf, nx, initializer_range=0.02, **kwargs):
         """ TFConv1D layer as defined by Radford et al. for OpenAI GPT (and also used in GPT-2)
@@ -849,7 +1118,7 @@ class TFSharedEmbeddings(tf.keras.layers.Layer):
         self.initializer_range = hidden_size ** -0.5 if initializer_range is None else initializer_range
 
     def build(self, input_shape):
-        """Build shared word embedding layer
+        """Build shared token embedding layer
         Shared weights logic adapted from
             https://github.com/tensorflow/models/blob/a009f4fb9d2fc4949e32192a944688925ef78659/official/transformer/v2/embedding_layer.py#L24
         """

tests/test_modeling_tf_common.py

@@ -381,7 +381,6 @@ class TFModelTesterMixin:
         )  # TODO (PVP): ugly workaround to make code work for t5 for the moment - has to changed when t5 is fixed.
 
         for model_class in self.all_generative_model_classes:
-            # TODO (PVP): add beam search tests when beam search is implemented
             model = model_class(config)
 
             if config.bos_token_id is None:
@@ -389,15 +388,34 @@ class TFModelTesterMixin:
                     model.generate(max_length=5)
                 # batch_size = 1
                 self._check_generated_tokens(model.generate(input_ids))
+                # batch_size = 1, num_beams > 1
+                self._check_generated_tokens(model.generate(input_ids, num_beams=3))
             else:
                 # batch_size = 1
                 self._check_generated_tokens(model.generate(max_length=5))
                 # batch_size = 1, num_beams > 1
+                self._check_generated_tokens(model.generate(max_length=5, num_beams=3))
+
+            with self.assertRaises(AssertionError):
+                # generating multiple sequences when greedy no beam generation
+                # is not allowed as it would always generate the same sequences
+                model.generate(input_ids, do_sample=False, num_return_sequences=2)
+
+            with self.assertRaises(AssertionError):
+                # generating more sequences than having beams leads is not possible
+                model.generate(input_ids, do_sample=False, num_return_sequences=3, num_beams=2)
 
             # batch_size > 1, sample
             self._check_generated_tokens(model.generate(input_ids, num_return_sequences=3))
             # batch_size > 1, greedy
-            self._check_generated_tokens(model.generate(input_ids, do_sample=False, num_return_sequences=3))
+            self._check_generated_tokens(model.generate(input_ids, do_sample=False))
+
+            # batch_size > 1, num_beams > 1, sample
+            self._check_generated_tokens(model.generate(input_ids, num_beams=3, num_return_sequences=3,))
+            # batch_size > 1, num_beams > 1, greedy
+            self._check_generated_tokens(
+                model.generate(input_ids, do_sample=False, num_beams=3, num_return_sequences=3)
+            )
 
     def _check_generated_tokens(self, output_ids):
         for token_id in output_ids[0].numpy().tolist():