def cond_fn(step_num, prev_ids, *unused_states):
"""Should we run another loop iteration."""
overflow = mtf.equal(step_num, num_steps)
has_eos = mtf.reduce_any(
mtf.equal(prev_ids, eos_id), reduced_dim=length_dim)
all_has_eos = mtf.reduce_all(has_eos)
return mtf.logical_not(mtf.logical_or(overflow, all_has_eos))
def _is_finished(i, unused_alive_seq, alive_log_probs, unused_finished_seq,
finished_scores, finished_in_finished, *unused_states):
"""Checking termination condition.
We terminate when we decoded up to decode_length or the lowest scoring item
in finished has a greater score that the highest prob item in alive divided
by the max length penalty
Args:
i: loop index
alive_log_probs: probabilities of the beams. [batch_size, beam_size]
finished_scores: scores for each of these sequences.
[batch_size, beam_size]
finished_in_finished: finished bools for each of these sequences.
[batch_size, beam_size]
Returns:
Bool.
"""
# TODO(noam): support a different decode length...
# decode_length = mtf.constant(mesh, length_dim.size, dtype=tf.int32)
# del alive_log_probs, finished_scores, finished_in_finished
# return mtf.less(i, length_dim.size)
if not stop_early:
return mtf.less(i, decode_length)
max_length_penalty = mtf.pow(
((5. + mtf.cast(decode_length, finished_scores.dtype)) / 6.),
alpha)
# The best possible score of the most likely alive sequence.
lower_bound_alive_scores = mtf.gather(
alive_log_probs, mtf.constant(mesh, 0, dtype=tf.int32),
beam_dim) / max_length_penalty
# Now to compute the lowest score of a finished sequence in finished
# If the sequence isn't finished, we multiply it's score by 0. since
# scores are all -ve, taking the min will give us the score of the lowest
# finished item.
lowest_score_of_finished_in_finished = mtf.reduce_min(
finished_scores *
mtf.cast(finished_in_finished, finished_scores.dtype),
reduced_dim=beam_dim)
# If none of the sequences have finished, then the min will be 0 and
# we have to replace it by -ve INF if it is. The score of any seq in alive
# will be much higher than -ve INF and the termination condition will not
# be met.
lowest_score_of_finished_in_finished += ((1. - mtf.cast(
mtf.reduce_any(finished_in_finished, reduced_dim=beam_dim),
finished_scores.dtype)) * -INF)
bound_is_met = mtf.reduce_all(
mtf.greater(lowest_score_of_finished_in_finished,
lower_bound_alive_scores))
return mtf.logical_and(mtf.less(i, decode_length),
mtf.logical_not(bound_is_met))
def beam_search(logits_fn,
initial_ids,
alpha,
states=None,
eos_id=EOS_ID,
stop_early=True,
decode_length=None,
use_tpu=True,
dtype=tf.float32):
"""Beam search with length penalties.
Requires a function that can take the currently decoded symbols and return
the logits for the next symbol. The implementation is inspired by
https://arxiv.org/abs/1609.08144.
When running, the beam search steps can be visualized by using tfdbg to watch
the operations generating the output ids for each beam step. These operations
have the pattern:
(alive|finished)_topk_(seq,scores)
Operations marked `alive` represent the new beam sequences that will be
processed in the next step. Operations marked `finished` represent the
completed beam sequences, which may be padded with 0s if no beams finished.
Operations marked `seq` store the full beam sequence for the time step.
Operations marked `scores` store the sequence's final log scores.
The beam search steps will be processed sequentially in order, so when
capturing observed from these operations, tensors, clients can make
assumptions about which step is being recorded.
WARNING: Assumes 2nd dimension of tensors in `states` and not invariant, this
means that the shape of the 2nd dimension of these tensors will not be
available (i.e. set to None) inside logits_fn.
Args:
logits_fn: Interface to the model, to provide logits.
Shoud take:
step_num - mtf Scalar
ids - mtf Tensor with shape [batch, beam, length]
Should return:
logits - [batch, beam, vocab_size], dtype=dtype
initial_ids: a mtf.Tensor with shape [batch_dim, beam_dim, length_dim])
alpha: alpha for length penalty.
states: list of mtf.Tensor
eos_id: ID for end of sentence.
stop_early: a boolean - stop once best sequence is provably determined.
decode_length: a mtf Scalar of dtype tf.int32 - maximum length of decodes
use_tpu: a boolean
dtype: a tf.dtype
Returns:
Tuple of
(decoded beams [batch, beam, length]
decoding probabilities [batch, beam_size])
"""
batch_dim, beam_dim, length_dim = initial_ids.shape.dims
mesh = initial_ids.mesh
batch_by_beam = mtf.Shape([batch_dim, beam_dim])
initial_log_probs = mtf.broadcast(
mtf.one_hot(
mtf.constant(mesh, 0, dtype=tf.int32),
beam_dim,
on_value=0.0,
off_value=-INF,
dtype=dtype),
batch_by_beam)
length_scalar = mtf.constant(mesh, length_dim.size, dtype=tf.int32)
if decode_length is None:
decode_length = length_scalar
else:
decode_length = mtf.minimum(decode_length, length_scalar)
alive_log_probs = initial_log_probs
alive_seq = initial_ids
# Finished will keep track of all the sequences that have finished so far
# Finished log probs will be negative infinity in the beginning
# finished_flags will keep track of booleans
finished_seq = initial_ids
finished_scores = mtf.constant(mesh, -INF, batch_by_beam, dtype=dtype)
# Setting the scores of the initial to negative infinity.
finished_flags = mtf.constant(mesh, False, batch_by_beam, tf.bool)
def grow_finished(finished_seq, finished_scores, finished_flags, curr_seq,
curr_scores, curr_finished):
"""Given sequences and scores, will gather the top k=beam size sequences.
Args:
finished_seq: Current finished sequences.
[batch, beam, length]
finished_scores: scores for each of these sequences.
[batch, beam]
finished_flags: finished bools for each of these sequences.
[batch, beam]
curr_seq: current topk sequence that has been grown by one position.
[batch, beam, length]
curr_scores: scores for each of these sequences. [batch, beam]
curr_finished: Finished flags for each of these sequences.
[batch, beam]
Returns:
Tuple of
(Topk sequences based on scores,
log probs of these sequences,
Finished flags of these sequences,
None (no states))
"""
# Set the scores of the unfinished seq in curr_seq to large negative
# values
curr_scores += (1. - mtf.cast(curr_finished, curr_scores.dtype)) * -INF
unused_batch_dim, beam_dim, unused_length_dim = finished_seq.shape.dims
# concatenating the sequences and scores along beam axis
def _my_concat(a, b):
a = mtf.rename_dimension(a, "beam", "triple_beam")
b = mtf.rename_dimension(b, "double_beam", "triple_beam")
return mtf.concat([a, b], "triple_beam")
curr_finished_seq = _my_concat(finished_seq, curr_seq)
curr_finished_scores = _my_concat(finished_scores, curr_scores)
curr_finished_flags = _my_concat(finished_flags, curr_finished)
return compute_topk_scores_and_seq(
curr_finished_seq, curr_finished_scores, curr_finished_scores,
curr_finished_flags, beam_dim, "grow_finished", states=None)
def grow_alive(curr_seq, curr_scores, curr_log_probs, curr_finished, states):
"""Given sequences and scores, will gather the top k=beam size sequences.
Args:
curr_seq: current topk sequence that has been grown by one position.
[batch, beam, length]
curr_scores: scores for each of these sequences. [batch_size, beam_size]
curr_log_probs: log probs for each of these sequences.
[batch, beam]
curr_finished: Finished flags for each of these sequences.
[batch, beam]
states: list of mtf.Tensor
Returns:
Tuple of
(Topk sequences based on scores,
log probs of these sequences,
Finished flags of these sequences)
"""
# Set the scores of the finished seq in curr_seq to large negative
# values
curr_scores += mtf.cast(curr_finished, curr_scores.dtype) * -INF
return compute_topk_scores_and_seq(curr_seq, curr_scores, curr_log_probs,
curr_finished, beam_dim,
"grow_alive", states)
def grow_topk(i, alive_seq, alive_log_probs, states=None):
r"""Inner beam search loop.
This function takes the current alive sequences, and grows them to topk
sequences where k = 2*beam. We use 2*beam because, we could have beam_size
number of sequences that might hit <EOS> and there will be no alive
sequences to continue. With 2*beam_size, this will not happen. This relies
on the assumption the vocab size is > beam size. If this is true, we'll
have at least beam_size non <EOS> extensions if we extract the next top
2*beam words.
Length penalty is given by = (5+len(decode)/6) ^ -\alpha. Pls refer to
https://arxiv.org/abs/1609.08144.
Args:
i: loop index
alive_seq: Topk sequences decoded so far [batch, beam, length]
alive_log_probs: probabilities of these sequences. [batch, beam]
states: optional list of mtf.Tensor
Returns:
Tuple of
(Topk sequences extended by the next word,
The log probs of these sequences,
The scores with length penalty of these sequences,
Flags indicating which of these sequences have finished decoding,
list of transformed decoding states)
"""
logits, new_states = logits_fn(i, alive_seq, states)
batch_dim, beam_dim, vocab_dim = logits.shape.dims
# Convert logits to normalized log probs
candidate_log_probs = mtf.log_softmax(logits, vocab_dim)
# Multiply the probabilities by the current probabilities of the beam.
# (batch_size, beam_size, vocab_size) + (batch_size, beam_size, 1)
log_probs = candidate_log_probs + alive_log_probs
length_penalty = mtf.pow(((5. + mtf.cast(i + 1, logits.dtype)) / 6.), alpha)
curr_scores = log_probs / length_penalty
# scores have shape [batch, beam, vocab]
beam_and_vocab_dim = mtf.Dimension(
"beam_and_vocab", beam_dim.size * vocab_dim.size)
flat_shape = mtf.Shape([batch_dim, beam_and_vocab_dim])
double_beam = mtf.Dimension("double_beam", beam_dim.size * 2)
# Flatten out (beam_size, vocab_size) probs in to a list of possibilities
flat_curr_scores = mtf.reshape(curr_scores, flat_shape)
top_ids, top_scores = mtf.top_k(
flat_curr_scores, reduced_dim=beam_and_vocab_dim, new_dim=double_beam)
# Recovering the log probs because we will need to send them back
top_log_probs = top_scores * length_penalty
# Work out what beam the top probs are in.
top_beam_index = top_ids // vocab_dim.size
top_ids %= vocab_dim.size # Unflatten the ids
def my_gather(tensor):
return mtf.gather(
tensor, top_beam_index, beam_dim,
output_shape=mtf.Shape(
[double_beam if d == beam_dim else d for d in tensor.shape.dims]))
# Gather up the most probable 2*beams both for the ids and finished_in_alive
# bools
top_seq = my_gather(alive_seq)
if states:
states = [my_gather(state) for state in new_states]
# Append the most probable alive
top_seq += top_ids * mtf.one_hot(i, length_dim, dtype=tf.int32)
top_finished = mtf.equal(top_ids, eos_id)
return top_seq, top_log_probs, top_scores, top_finished, states
def inner_loop(i, alive_seq, alive_log_probs, finished_seq, finished_scores,
finished_flags, *states):
"""Inner beam search loop.
There are three groups of tensors, alive, finished, and topk.
The alive group contains information about the current alive sequences
The topk group contains information about alive + topk current decoded words
the finished group contains information about finished sentences, that is,
the ones that have decoded to <EOS>. These are what we return.
The general beam search algorithm is as follows:
While we haven't terminated (pls look at termination condition)
1. Grow the current alive to get beam*2 topk sequences
2. Among the topk, keep the top beam_size ones that haven't reached EOS
into alive
3. Among the topk, keep the top beam_size ones have reached EOS into
finished
Repeat
To make things simple with using fixed size tensors, we will end
up inserting unfinished sequences into finished in the beginning. To stop
that we add -ve INF to the score of the unfinished sequence so that when a
true finished sequence does appear, it will have a higher score than all the
unfinished ones.
Args:
i: loop index
alive_seq: Topk sequences decoded so far [batch_size, beam_size, i+1]
alive_log_probs: probabilities of the beams. [batch_size, beam_size]
finished_seq: Current finished sequences.
[batch_size, beam_size, i+1]
finished_scores: scores for each of these sequences.
[batch_size, beam_size]
finished_flags: finished bools for each of these sequences.
[batch_size, beam_size]
*states: mtf Tensors
Returns:
Tuple of
(Incremented loop index
New alive sequences,
Log probs of the alive sequences,
New finished sequences,
Scores of the new finished sequences,
Flags indicating which sequence in finished as reached EOS,
dict of final decoding states)
"""
# Each inner loop, we carry out three steps:
# 1. Get the current topk items.
# 2. Extract the ones that have finished and haven't finished
# 3. Recompute the contents of finished based on scores.
(top2k_seq, top2k_log_probs, top2k_scores, top2k_finished,
top2k_states) = grow_topk(i, alive_seq, alive_log_probs, states)
alive_seq, alive_log_probs, _, states = grow_alive(
top2k_seq, top2k_scores, top2k_log_probs, top2k_finished, top2k_states)
finished_seq, finished_scores, finished_flags, _ = grow_finished(
finished_seq, finished_scores, finished_flags, top2k_seq, top2k_scores,
top2k_finished)
return (i + 1, alive_seq, alive_log_probs, finished_seq, finished_scores,
finished_flags) + tuple(states)
def _is_finished(i, unused_alive_seq, alive_log_probs, unused_finished_seq,
finished_scores, finished_in_finished, *unused_states):
"""Checking termination condition.
We terminate when we decoded up to decode_length or the lowest scoring item
in finished has a greater score that the highest prob item in alive divided
by the max length penalty
Args:
i: loop index
alive_log_probs: probabilities of the beams. [batch_size, beam_size]
finished_scores: scores for each of these sequences.
[batch_size, beam_size]
finished_in_finished: finished bools for each of these sequences.
[batch_size, beam_size]
Returns:
Bool.
"""
# TODO(noam): support a different decode length...
# decode_length = mtf.constant(mesh, length_dim.size, dtype=tf.int32)
# del alive_log_probs, finished_scores, finished_in_finished
# return mtf.less(i, length_dim.size)
if not stop_early:
return mtf.less(i, decode_length)
max_length_penalty = mtf.pow(
((5. + mtf.cast(decode_length, finished_scores.dtype)) / 6.), alpha)
# The best possible score of the most likely alive sequence.
lower_bound_alive_scores = mtf.gather(
alive_log_probs, mtf.constant(mesh, 0, dtype=tf.int32),
beam_dim) / max_length_penalty
# Now to compute the lowest score of a finished sequence in finished
# If the sequence isn't finished, we multiply it's score by 0. since
# scores are all -ve, taking the min will give us the score of the lowest
# finished item.
lowest_score_of_finished_in_finished = mtf.reduce_min(
finished_scores * mtf.cast(finished_in_finished, finished_scores.dtype),
reduced_dim=beam_dim)
# If none of the sequences have finished, then the min will be 0 and
# we have to replace it by -ve INF if it is. The score of any seq in alive
# will be much higher than -ve INF and the termination condition will not
# be met.
lowest_score_of_finished_in_finished += (
(1. - mtf.cast(mtf.reduce_any(
finished_in_finished, reduced_dim=beam_dim),
finished_scores.dtype)) * -INF)
bound_is_met = mtf.reduce_all(
mtf.greater(lowest_score_of_finished_in_finished,
lower_bound_alive_scores))
return mtf.logical_and(
mtf.less(i, decode_length), mtf.logical_not(bound_is_met))
initial_step_num = mtf.constant(mesh, 0, dtype=tf.int32)
while_loop_inputs = [
initial_step_num, alive_seq, alive_log_probs, finished_seq,
finished_scores, finished_flags] + states
(_, alive_seq, alive_log_probs, finished_seq, finished_scores,
finished_flags) = mtf.while_loop(
_is_finished, inner_loop, while_loop_inputs,
num_loop_vars=None if use_tpu else 6)[:6]
# Accounting for corner case: It's possible that no sequence in alive for a
# particular batch item ever reached EOS. In that case, we should just copy
# the contents of alive for that batch item. tf.reduce_any(finished_flags, 1)
# if 0, means that no sequence for that batch index had reached EOS. We need
# to do the same for the scores as well.
finished_seq = mtf.where(
mtf.reduce_any(finished_flags, reduced_dim=beam_dim),
finished_seq, alive_seq)
finished_scores = mtf.where(
mtf.reduce_any(finished_flags, reduced_dim=beam_dim),
finished_scores, alive_log_probs)
return finished_seq, finished_scores
