Source code for onmt.translate.decode_strategy

import torch
from copy import deepcopy

from onmt.utils.misc import tile


[docs]class DecodeStrategy(object): """Base class for generation strategies. Args: pad (int): Magic integer in output vocab. bos (int): Magic integer in output vocab. eos (int): Magic integer in output vocab. unk (int): Magic integer in output vocab. batch_size (int): Current batch size. parallel_paths (int): Decoding strategies like beam search use parallel paths. Each batch is repeated ``parallel_paths`` times in relevant state tensors. min_length (int): Shortest acceptable generation, not counting begin-of-sentence or end-of-sentence. max_length (int): Longest acceptable sequence, not counting begin-of-sentence (presumably there has been no EOS yet if max_length is used as a cutoff). ban_unk_token (Boolean): Whether unk token is forbidden block_ngram_repeat (int): Block beams where ``block_ngram_repeat``-grams repeat. exclusion_tokens (set[int]): If a gram contains any of these tokens, it may repeat. return_attention (bool): Whether to work with attention too. If this is true, it is assumed that the decoder is attentional. Attributes: pad (int): See above. bos (int): See above. eos (int): See above. unk (int): See above. predictions (list[list[LongTensor]]): For each batch, holds a list of beam prediction sequences. scores (list[list[FloatTensor]]): For each batch, holds a list of scores. attention (list[list[FloatTensor or list[]]]): For each batch, holds a list of attention sequence tensors (or empty lists) having shape ``(step, inp_seq_len)`` where ``inp_seq_len`` is the length of the sample (not the max length of all inp seqs). alive_seq (LongTensor): Shape ``(B x parallel_paths, step)``. This sequence grows in the ``step`` axis on each call to :func:`advance()`. is_finished (ByteTensor or NoneType): Shape ``(B, parallel_paths)``. Initialized to ``None``. alive_attn (FloatTensor or NoneType): If tensor, shape is ``(step, B x parallel_paths, inp_seq_len)``, where ``inp_seq_len`` is the (max) length of the input sequence. target_prefix (LongTensor or NoneType): If tensor, shape is ``(B x parallel_paths, prefix_seq_len)``, where ``prefix_seq_len`` is the (max) length of the pre-fixed prediction. min_length (int): See above. max_length (int): See above. ban_unk_token (Boolean): See above. block_ngram_repeat (int): See above. exclusion_tokens (set[int]): See above. return_attention (bool): See above. done (bool): See above. """ def __init__(self, pad, bos, eos, unk, batch_size, parallel_paths, global_scorer, min_length, block_ngram_repeat, exclusion_tokens, return_attention, max_length, ban_unk_token): # magic indices self.pad = pad self.bos = bos self.eos = eos self.unk = unk self.batch_size = batch_size self.parallel_paths = parallel_paths self.global_scorer = global_scorer # result caching self.predictions = [[] for _ in range(batch_size)] self.scores = [[] for _ in range(batch_size)] self.attention = [[] for _ in range(batch_size)] self.hypotheses = [[] for _ in range(batch_size)] self.alive_attn = None self.min_length = min_length self.max_length = max_length self.ban_unk_token = ban_unk_token self.block_ngram_repeat = block_ngram_repeat n_paths = batch_size * parallel_paths self.forbidden_tokens = [dict() for _ in range(n_paths)] self.exclusion_tokens = exclusion_tokens self.return_attention = return_attention self.done = False def get_device_from_memory_bank(self, memory_bank): if isinstance(memory_bank, tuple): mb_device = memory_bank[0].device else: mb_device = memory_bank.device return mb_device def initialize_tile(self, memory_bank, src_lengths, src_map=None, target_prefix=None): def fn_map_state(state, dim): return tile(state, self.beam_size, dim=dim) if isinstance(memory_bank, tuple): memory_bank = tuple(tile(x, self.beam_size, dim=1) for x in memory_bank) elif memory_bank is not None: memory_bank = tile(memory_bank, self.beam_size, dim=1) if src_map is not None: src_map = tile(src_map, self.beam_size, dim=1) self.memory_lengths = tile(src_lengths, self.beam_size) if target_prefix is not None: target_prefix = tile(target_prefix, self.beam_size, dim=1) return fn_map_state, memory_bank, src_map, target_prefix
[docs] def initialize(self, memory_bank, src_lengths, src_map=None, device=None, target_prefix=None): """DecodeStrategy subclasses should override :func:`initialize()`. `initialize` should be called before all actions. used to prepare necessary ingredients for decode. """ if device is None: device = torch.device('cpu') self.alive_seq = torch.full( [self.batch_size * self.parallel_paths, 1], self.bos, dtype=torch.long, device=device) self.is_finished = torch.zeros( [self.batch_size, self.parallel_paths], dtype=torch.uint8, device=device) if target_prefix is not None: seq_len, batch_size, n_feats = target_prefix.size() assert batch_size == self.batch_size * self.parallel_paths,\ "forced target_prefix should've extend to same number of path!" target_prefix_words = target_prefix[:, :, 0].transpose(0, 1) target_prefix = target_prefix_words[:, 1:] # remove bos # fix length constraint and remove eos from count prefix_non_pad = target_prefix.ne(self.pad).sum(dim=-1).tolist() self.max_length += max(prefix_non_pad)-1 self.min_length += min(prefix_non_pad)-1 self.target_prefix = target_prefix # NOTE: forced prefix words return None, memory_bank, src_lengths, src_map
def __len__(self): return self.alive_seq.shape[1] def ensure_min_length(self, log_probs): if len(self) <= self.min_length: log_probs[:, self.eos] = -1e20 def ensure_unk_removed(self, log_probs): if self.ban_unk_token: log_probs[:, self.unk] = -1e20 def ensure_max_length(self): # add one to account for BOS. Don't account for EOS because hitting # this implies it hasn't been found. if len(self) == self.max_length + 1: self.is_finished.fill_(1)
[docs] def block_ngram_repeats(self, log_probs): """ We prevent the beam from going in any direction that would repeat any ngram of size <block_ngram_repeat> more thant once. The way we do it: we maintain a list of all ngrams of size <block_ngram_repeat> that is updated each time the beam advances, and manually put any token that would lead to a repeated ngram to 0. This improves on the previous version's complexity: - previous version's complexity: batch_size * beam_size * len(self) - current version's complexity: batch_size * beam_size This improves on the previous version's accuracy; - Previous version blocks the whole beam, whereas here we only block specific tokens. - Before the translation would fail when all beams contained repeated ngrams. This is sure to never happen here. """ # we don't block nothing if the user doesn't want it if self.block_ngram_repeat <= 0: return # we can't block nothing beam's too short if len(self) < self.block_ngram_repeat: return n = self.block_ngram_repeat - 1 for path_idx in range(self.alive_seq.shape[0]): # we check paths one by one current_ngram = tuple(self.alive_seq[path_idx, -n:].tolist()) forbidden_tokens = self.forbidden_tokens[path_idx].get( current_ngram, None) if forbidden_tokens is not None: log_probs[path_idx, list(forbidden_tokens)] = -10e20
[docs] def maybe_update_forbidden_tokens(self): """We complete and reorder the list of forbidden_tokens""" # we don't forbid nothing if the user doesn't want it if self.block_ngram_repeat <= 0: return # we can't forbid nothing if beam's too short if len(self) < self.block_ngram_repeat: return n = self.block_ngram_repeat forbidden_tokens = list() for path_idx, seq in zip(self.select_indices, self.alive_seq): # Reordering forbidden_tokens following beam selection # We rebuild a dict to ensure we get the value and not the pointer forbidden_tokens.append( deepcopy(self.forbidden_tokens[path_idx])) # Grabing the newly selected tokens and associated ngram current_ngram = tuple(seq[-n:].tolist()) # skip the blocking if any token in current_ngram is excluded if set(current_ngram) & self.exclusion_tokens: continue forbidden_tokens[-1].setdefault(current_ngram[:-1], set()) forbidden_tokens[-1][current_ngram[:-1]].add(current_ngram[-1]) self.forbidden_tokens = forbidden_tokens
[docs] def target_prefixing(self, log_probs): """Fix the first part of predictions with `self.target_prefix`. Args: log_probs (FloatTensor): logits of size ``(B, vocab_size)``. Returns: log_probs (FloatTensor): modified logits in ``(B, vocab_size)``. """ _B, vocab_size = log_probs.size() step = len(self) if (self.target_prefix is not None and step <= self.target_prefix.size(1)): pick_idx = self.target_prefix[:, step - 1].tolist() # (B) pick_coo = [[path_i, pick] for path_i, pick in enumerate(pick_idx) if pick not in [self.eos, self.pad]] mask_pathid = [path_i for path_i, pick in enumerate(pick_idx) if pick in [self.eos, self.pad]] if len(pick_coo) > 0: pick_coo = torch.tensor(pick_coo).to(self.target_prefix) pick_fill_value = torch.ones( [pick_coo.size(0)], dtype=log_probs.dtype) # pickups: Tensor where specified index were set to 1, others 0 pickups = torch.sparse_coo_tensor( pick_coo.t(), pick_fill_value, size=log_probs.size(), device=log_probs.device).to_dense() # dropdowns: opposite of pickups, 1 for those shouldn't pick dropdowns = torch.ones_like(pickups) - pickups if len(mask_pathid) > 0: path_mask = torch.zeros(_B).to(self.target_prefix) path_mask[mask_pathid] = 1 path_mask = path_mask.unsqueeze(1).to(dtype=bool) dropdowns = dropdowns.masked_fill(path_mask, 0) # Minus dropdowns to log_probs making probabilities of # unspecified index close to 0 log_probs -= 10000*dropdowns return log_probs
[docs] def maybe_update_target_prefix(self, select_index): """We update / reorder `target_prefix` for alive path.""" if self.target_prefix is None: return # prediction step have surpass length of given target_prefix, # no need to further change this attr if len(self) > self.target_prefix.size(1): return self.target_prefix = self.target_prefix.index_select(0, select_index)
[docs] def advance(self, log_probs, attn): """DecodeStrategy subclasses should override :func:`advance()`. Advance is used to update ``self.alive_seq``, ``self.is_finished``, and, when appropriate, ``self.alive_attn``. """ raise NotImplementedError()
[docs] def update_finished(self): """DecodeStrategy subclasses should override :func:`update_finished()`. ``update_finished`` is used to update ``self.predictions``, ``self.scores``, and other "output" attributes. """ raise NotImplementedError()