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# Copyright (c) Facebook, Inc. and its affiliates.
#
# This source code is licensed under the MIT license found in the
# LICENSE file in the root directory of this source tree.
from functools import lru_cache
import numpy as np
import torch
from fairseq.data import Dictionary, data_utils
from . import BaseWrapperDataset, LRUCacheDataset
class MaskTokensDataset(BaseWrapperDataset):
"""
A wrapper Dataset for masked language modeling.
Input items are masked according to the specified masking probability.
Args:
dataset: Dataset to wrap.
sizes: Sentence lengths
vocab: Dictionary with the vocabulary and special tokens.
pad_idx: Id of pad token in vocab
mask_idx: Id of mask token in vocab
return_masked_tokens: controls whether to return the non-masked tokens
(the default) or to return a tensor with the original masked token
IDs (and *pad_idx* elsewhere). The latter is useful as targets for
masked LM training.
seed: Seed for random number generator for reproducibility.
mask_prob: probability of replacing a token with *mask_idx*.
leave_unmasked_prob: probability that a masked token is unmasked.
random_token_prob: probability of replacing a masked token with a
random token from the vocabulary.
freq_weighted_replacement: sample random replacement words based on
word frequencies in the vocab.
mask_whole_words: only mask whole words. This should be a byte mask
over vocab indices, indicating whether it is the beginning of a
word. We will extend any mask to encompass the whole word.
bpe: BPE to use for whole-word masking.
mask_multiple_length : repeat each mask index multiple times. Default
value is 1.
mask_stdev : standard deviation of masks distribution in case of
multiple masking. Default value is 0.
"""
@classmethod
def apply_mask(cls, dataset: torch.utils.data.Dataset, *args, **kwargs):
"""Return the source and target datasets for masked LM training."""
dataset = LRUCacheDataset(dataset)
return (
LRUCacheDataset(cls(dataset, *args, **kwargs, return_masked_tokens=False)),
LRUCacheDataset(cls(dataset, *args, **kwargs, return_masked_tokens=True)),
)
def __init__(
self,
dataset: torch.utils.data.Dataset,
vocab: Dictionary,
pad_idx: int,
mask_idx: int,
return_masked_tokens: bool = False,
seed: int = 1,
mask_prob: float = 0.15,
leave_unmasked_prob: float = 0.1,
random_token_prob: float = 0.1,
freq_weighted_replacement: bool = False,
mask_whole_words: torch.Tensor = None,
mask_multiple_length: int = 1,
mask_stdev: float = 0.0,
skip_masking: bool = False,
):
assert 0.0 < mask_prob < 1.0
assert 0.0 <= random_token_prob <= 1.0
assert 0.0 <= leave_unmasked_prob <= 1.0
assert random_token_prob + leave_unmasked_prob <= 1.0
assert mask_multiple_length >= 1
assert mask_stdev >= 0.0
self.dataset = dataset
self.vocab = vocab
self.pad_idx = pad_idx
self.mask_idx = mask_idx
self.return_masked_tokens = return_masked_tokens
self.seed = seed
self.mask_prob = mask_prob
self.leave_unmasked_prob = leave_unmasked_prob
self.random_token_prob = random_token_prob
self.mask_whole_words = mask_whole_words
self.mask_multiple_length = mask_multiple_length
self.mask_stdev = mask_stdev
self.skip_masking = skip_masking
if random_token_prob > 0.0:
if freq_weighted_replacement:
weights = np.array(self.vocab.count)
else:
weights = np.ones(len(self.vocab))
weights[: self.vocab.nspecial] = 0
self.weights = weights / weights.sum()
self.epoch = 0
@property
def can_reuse_epoch_itr_across_epochs(self):
return True # only the noise changes, not item sizes
def set_epoch(self, epoch, **unused):
super().set_epoch(epoch)
self.epoch = epoch
def __getitem__(self, index: int):
return self.__getitem_cached__(self.seed, self.epoch, index)
@lru_cache(maxsize=8)
def __getitem_cached__(self, seed: int, epoch: int, index: int):
seed = int(hash((seed, epoch, index)) % 1e6)
rng = np.random.default_rng(seed)
item = self.dataset[index]
sz = len(item)
assert (
self.mask_idx not in item
), "Dataset contains mask_idx (={}), this is not expected!".format(
self.mask_idx,
)
if self.skip_masking:
return torch.from_numpy(np.copy(item))
if self.mask_whole_words is not None:
word_begins_mask = self.mask_whole_words.gather(0, item)
word_begins_idx = word_begins_mask.nonzero().view(-1)
sz = len(word_begins_idx)
words = np.split(word_begins_mask, word_begins_idx)[1:]
assert len(words) == sz
word_lens = list(map(len, words))
# decide elements to mask
mask = np.full(sz, False)
num_mask = int(
# add a random number for probabilistic rounding
self.mask_prob * sz / float(self.mask_multiple_length)
+ rng.random()
)
# multiple masking as described in the vq-wav2vec paper (https://arxiv.org/abs/1910.05453)
mask_idc = rng.choice(sz, num_mask, replace=False)
if self.mask_stdev > 0.0:
lengths = rng.normal(
self.mask_multiple_length, self.mask_stdev, size=num_mask
)
lengths = [max(0, int(round(x))) for x in lengths]
mask_idc = np.asarray(
[
mask_idc[j] + offset
for j in range(len(mask_idc))
for offset in range(lengths[j])
],
dtype=np.int64,
)
else:
mask_idc = np.concatenate(
[mask_idc + i for i in range(self.mask_multiple_length)]
)
mask_idc = mask_idc[mask_idc < len(mask)]
try:
mask[mask_idc] = True
except: # something wrong
print("Assigning mask indexes {} to mask {} failed!".format(mask_idc, mask))
raise
# if self.return_masked_tokens:
# print((
# f"IDX={index}; seed={seed}; epoch={epoch}; is_tgt={self.return_masked_tokens}: "
# f"{np.nonzero(mask)[0].sum()}"
# ))
if self.return_masked_tokens:
# exit early if we're just returning the masked tokens
# (i.e., the targets for masked LM training)
if self.mask_whole_words is not None:
mask = np.repeat(mask, word_lens)
new_item = np.full(len(mask), self.pad_idx)
new_item[mask] = item[torch.from_numpy(mask.astype(np.uint8)) == 1]
return torch.from_numpy(new_item)
# decide unmasking and random replacement
rand_or_unmask_prob = self.random_token_prob + self.leave_unmasked_prob
if rand_or_unmask_prob > 0.0:
rand_or_unmask = mask & (rng.random(sz) < rand_or_unmask_prob)
if self.random_token_prob == 0.0:
unmask = rand_or_unmask
rand_mask = None
elif self.leave_unmasked_prob == 0.0:
unmask = None
rand_mask = rand_or_unmask
else:
unmask_prob = self.leave_unmasked_prob / rand_or_unmask_prob
decision = rng.random(sz) < unmask_prob
unmask = rand_or_unmask & decision
rand_mask = rand_or_unmask & (~decision)
else:
unmask = rand_mask = None
if unmask is not None:
mask = mask ^ unmask
if self.mask_whole_words is not None:
mask = np.repeat(mask, word_lens)
new_item = np.copy(item)
new_item[mask] = self.mask_idx
if rand_mask is not None:
num_rand = rand_mask.sum()
if num_rand > 0:
if self.mask_whole_words is not None:
rand_mask = np.repeat(rand_mask, word_lens)
num_rand = rand_mask.sum()
new_item[rand_mask] = rng.choice(
len(self.vocab),
num_rand,
p=self.weights,
)
return torch.from_numpy(new_item)
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