Tokenizer summary (#5467)
* Work on tokenizer summary * Finish tutorial * Link to it * Apply suggestions from code review Co-authored-by: Anthony MOI <xn1t0x@gmail.com> Co-authored-by: Lysandre Debut <lysandre@huggingface.co> * Add vocab definition Co-authored-by: Anthony MOI <xn1t0x@gmail.com> Co-authored-by: Lysandre Debut <lysandre@huggingface.co>
This commit is contained in:
parent
ef0e9d806c
commit
6b735a7253
|
@ -142,6 +142,7 @@ conversion utilities for the following models:
|
|||
preprocessing
|
||||
training
|
||||
model_sharing
|
||||
tokenizer_summary
|
||||
multilingual
|
||||
|
||||
.. toctree::
|
||||
|
|
|
@ -146,8 +146,9 @@ Using the tokenizer
|
|||
|
||||
We mentioned the tokenizer is responsible for the preprocessing of your texts. First, it will split a given text in
|
||||
words (or part of words, punctuation symbols, etc.) usually called `tokens`. There are multiple rules that can govern
|
||||
that process, which is why we need to instantiate the tokenizer using the name of the model, to make sure we use the
|
||||
same rules as when the model was pretrained.
|
||||
that process (you can learn more about them in the :doc:`tokenizer_summary <tokenizer_summary>`, which is why we need
|
||||
to instantiate the tokenizer using the name of the model, to make sure we use the same rules as when the model was
|
||||
pretrained.
|
||||
|
||||
The second step is to convert those `tokens` into numbers, to be able to build a tensor out of them and feed them to
|
||||
the model. To do this, the tokenizer has a `vocab`, which is the part we download when we instantiate it with the
|
||||
|
|
|
@ -0,0 +1,243 @@
|
|||
Tokenizer summary
|
||||
-----------------
|
||||
|
||||
In this page, we will have a closer look at tokenization. As we saw in
|
||||
:doc:`the preprocessing tutorial <preprocessing>`, tokenizing a text is splitting it into words or subwords, which then
|
||||
are converted to ids. The second part is pretty straightforward, here we will focus on the first part. More
|
||||
specifically, we will look at the three main different kinds of tokenizers used in 🤗 Transformers:
|
||||
:ref:`Byte-Pair Encoding (BPE) <byte-pair-encoding>`, :ref:`WordPiece <wordpiece>` and
|
||||
:ref:`SentencePiece <sentencepiece>`, and provide examples of models using each of those.
|
||||
|
||||
Note that on each model page, you can look at the documentation of the associated tokenizer to know which of those
|
||||
algorithms the pretrained model used. For instance, if we look at :class:`~transformers.BertTokenizer`, we can see it's
|
||||
using :ref:`WordPiece <wordpiece>`.
|
||||
|
||||
Introduction to tokenization
|
||||
~~~~~~~~~~~~~~~~~~~~~~~~~~~~
|
||||
|
||||
Splitting a text in smaller chunks is a task that's harder than it looks, and there are multiple ways of doing it. For
|
||||
instance, let's look at the sentence "Don't you love 🤗 Transformers? We sure do." A first simple way of tokenizing
|
||||
this text is just to split it by spaces, which would give:
|
||||
|
||||
::
|
||||
|
||||
["Don't", "you", "love", "🤗", "Transformers?", "We", "sure", "do."]
|
||||
|
||||
This is a nice first step, but if we look at the tokens "Transformers?" or "do.", we can see we can do better. Those
|
||||
will be different than the tokens "Transformers" and "do" for our model, so we should probably take the punctuation
|
||||
into account. This would give:
|
||||
|
||||
::
|
||||
|
||||
["Don", "'", "t", "you", "love", "🤗", "Transformers", "?", "We", "sure", "do", "."]
|
||||
|
||||
which is better already. One thing that is annoying though is how it dealt with "Don't". "Don't" stands for do not, so
|
||||
it should probably be better tokenized as ``["Do", "n't"]``. This is where things start getting more complicated, and
|
||||
part of the reason each kind of model has its own tokenizer class. Depending on the rules we apply to split our texts
|
||||
into tokens, we'll get different tokenized versions of the same text. And of course, a given pretrained model won't
|
||||
perform properly if you don't use the exact same rules as the persons who pretrained it.
|
||||
|
||||
`spaCy <https://spacy.io/>`__ and `Moses <http://www.statmt.org/moses/?n=Development.GetStarted>`__ are two popular
|
||||
rule-based tokenizers. On the text above, they'd output something like:
|
||||
|
||||
::
|
||||
|
||||
["Do", "n't", "you", "love", "🤗", "Transformers", "?", "We", "sure", "do", "."]
|
||||
|
||||
Space/punctuation-tokenization and rule-based tokenization are both examples of word tokenization, which is splitting a
|
||||
sentence into words. While it's the most intuitive way to separate texts in smaller chunks, it can have a problem when
|
||||
you have a huge corpus: it usually yields a very big vocabulary (the set of all unique tokens used).
|
||||
:doc:`Transformer XL <model_doc/transformerxl>` for instance uses space/punctuation-tokenization, and has a vocabulary
|
||||
size of 267,735!
|
||||
|
||||
A huge vocabulary size means a huge embedding matrix at the start of the model, which will cause memory problems.
|
||||
TransformerXL deals with it by using a special kind of embeddings called adaptive embeddings, but in general,
|
||||
transformers model rarely have a vocabulary size greater than 50,000, especially if they are trained on a single
|
||||
language.
|
||||
|
||||
So if tokenizing on words is unsatisfactory, we could go on the opposite direction and simply tokenize on characters.
|
||||
While it's very simple and would save a lot of memory, this doesn't allow the model to learn representations of texts
|
||||
as meaningful as when using a word tokenization, leading to a loss of performance. So to get the best of both worlds,
|
||||
all transformers models use a hybrid between word-level and character-level tokenization called subword tokenization.
|
||||
|
||||
Subword tokenization
|
||||
^^^^^^^^^^^^^^^^^^^^
|
||||
|
||||
Subword tokenization algorithms rely on the principle that most common words should be left as is, but rare words
|
||||
should be decomposed in meaningful subword units. For instance "annoyingly" might be considered a rare word and
|
||||
decomposed as "annoying" and "ly". This is especially useful in agglutinative languages such as Turkish, where you can
|
||||
form (almost) arbitrarily long complex words by stringing together some subwords.
|
||||
|
||||
This allows the model to keep a reasonable vocabulary while still learning useful representations for common words or
|
||||
subwords. This also gives the ability to the model to process words it has never seen before, by decomposing them into
|
||||
subwords it knows. For instance, the base :class:`~transformers.BertTokenizer` will tokenize "I have a new GPU!" like
|
||||
this:
|
||||
|
||||
::
|
||||
|
||||
>>> from transformers import BertTokenizer
|
||||
>>> tokenizer = BertTokenizer.from_pretrained('bert-base-uncased')
|
||||
>>> tokenizer.tokenize("I have a new GPU!")
|
||||
['i', 'have', 'a', 'new', 'gp', '##u', '!']
|
||||
|
||||
Since we are considering the uncased model, the sentence was lowercased first. Then all the words were present in the
|
||||
vocabulary of the tokenizer, except for "gpu", so the tokenizer split it in subwords it knows: "gp" and "##u". The "##"
|
||||
means that the rest of the token should be attached to the previous one, without space (for when we need to decode
|
||||
predictions and reverse the tokenization).
|
||||
|
||||
Another example is when we use the base :class:`~transformers.XLNetTokenizer` to tokenize our previous text:
|
||||
|
||||
::
|
||||
|
||||
>>> from transformers import XLNetTokenizer
|
||||
>>> tokenizer = XLNetTokenizer.from_pretrained('xlnet-base-cased')
|
||||
>>> tokenizer.tokenize("Don't you love 🤗 Transformers? We sure do.")
|
||||
['▁Don', "'", 't', '▁you', '▁love', '▁', '🤗', '▁', 'Transform', 'ers', '?', '▁We', '▁sure', '▁do', '.']
|
||||
|
||||
We'll get back to the meaning of those '▁' when we look at :ref:`SentencePiece <sentencepiece>` but you can see
|
||||
Transformers has been split into "Transform" and "ers".
|
||||
|
||||
Let's now look at how the different subword tokenization algorithms work. Note that they all rely on some form of
|
||||
training which is usually done on the corpus the corresponding model will be trained on.
|
||||
|
||||
.. _byte-pair-encoding:
|
||||
|
||||
Byte-Pair Encoding
|
||||
~~~~~~~~~~~~~~~~~~
|
||||
|
||||
Byte-Pair Encoding was introduced in `this paper <https://arxiv.org/abs/1508.07909>`__. It relies on a pretokenizer
|
||||
splitting the training data into words, which can be a simple space tokenization
|
||||
(:doc:`GPT-2 <model_doc/gpt2>` and :doc:`Roberta <model_doc/roberta>` uses this for instance) or a rule-based tokenizer
|
||||
(:doc:`XLM <model_doc/xlm>` use Moses for most languages, as does :doc:`FlauBERT <model_doc/flaubert>`),
|
||||
|
||||
:doc:`GPT <model_doc/gpt>` uses Spacy and ftfy) and, counts the frequency of each word in the training corpus.
|
||||
|
||||
It then begins from the list of all characters, and will learn merge rules to form a new token from two symbols in the
|
||||
vocabulary until it has learned a vocabulary of the desired size (this is a hyperparameter to pick).
|
||||
|
||||
Let's say that after the pre-tokenization we have the following words (the number indicating the frequency of each
|
||||
word):
|
||||
|
||||
::
|
||||
|
||||
('hug', 10), ('pug', 5), ('pun', 12), ('bun', 4), ('hugs', 5)
|
||||
|
||||
Then the base vocabulary is ['b', 'g', 'h', 'n', 'p', 's', 'u'] and all our words are first split by character:
|
||||
|
||||
::
|
||||
|
||||
('h' 'u' 'g', 10), ('p' 'u' 'g', 5), ('p' 'u' 'n', 12), ('b' 'u' 'n', 4), ('h' 'u' 'g' 's', 5)
|
||||
|
||||
We then take each pair of symbols and look at the most frequent. For instance 'hu' is present `10 + 5 = 15` times (10
|
||||
times in the 10 occurrences of 'hug', 5 times in the 5 occurrences of 'hugs'). The most frequent here is 'ug', present
|
||||
`10 + 5 + 2 + 5 = 22` times in total. So the first merge rule the tokenizer learns is to group all 'u' and 'g' together
|
||||
then it adds 'ug' to the vocabulary. Our corpus then becomes
|
||||
|
||||
::
|
||||
|
||||
('h' 'ug', 10), ('p' 'ug', 5), ('p' 'u' 'n', 12), ('b' 'u' 'n', 4), ('h' 'ug' 's', 5)
|
||||
|
||||
and we continue by looking at the next most common pair of symbols. It's 'un', present 16 times, so we merge those two
|
||||
and add 'un' to the vocabulary. Then it's 'hug' (as 'h' + 'ug'), present 15 times, so we merge those two and add 'hug'
|
||||
to the vocabulary.
|
||||
|
||||
At this stage, the vocabulary is ``['b', 'g', 'h', 'n', 'p', 's', 'u', 'ug', 'un', 'hug']`` and our corpus is
|
||||
represented as
|
||||
|
||||
::
|
||||
|
||||
('hug', 10), ('p' 'ug', 5), ('p' 'un', 12), ('b' 'un', 4), ('hug' 's', 5)
|
||||
|
||||
If we stop there, the tokenizer can apply the rules it learned to new words (as long as they don't contain characters that
|
||||
were not in the base vocabulary). For instance 'bug' would be tokenized as ``['b', 'ug']`` but mug would be tokenized as
|
||||
``['<unk>', 'ug']`` since the 'm' is not in the base vocabulary. This doesn't happen to letters in general (since the
|
||||
base corpus uses all of them), but to special characters like emojis.
|
||||
|
||||
As we said before, the vocabulary size (which is the base vocabulary size + the number of merges) is a hyperparameter
|
||||
to choose. For instance :doc:`GPT <model_doc/gpt>` has a vocabulary size of 40,478 since they have 478 base characters
|
||||
and chose to stop the training of the tokenizer at 40,000 merges.
|
||||
|
||||
Byte-level BPE
|
||||
^^^^^^^^^^^^^^
|
||||
|
||||
To deal with the fact the base vocabulary needs to get all base characters, which can be quite big if one allows for
|
||||
all unicode characters, the
|
||||
`GPT-2 paper <https://cdn.openai.com/better-language-models/language_models_are_unsupervised_multitask_learners.pdf>`__
|
||||
introduces a clever trick, which is to use bytes as the base vocabulary (which gives a size of 256). With some
|
||||
additional rules to deal with punctuation, this manages to be able to tokenize every text without needing an unknown
|
||||
token. For instance, the :doc:`GPT-2 model <model_doc/gpt>` has a vocabulary size of 50,257, which corresponds to the
|
||||
256 bytes base tokens, a special end-of-text token and the symbols learned with 50,000 merges.
|
||||
|
||||
.. _wordpiece:
|
||||
|
||||
WordPiece
|
||||
=========
|
||||
|
||||
WordPiece is the subword tokenization algorithm used for :doc:`BERT <model_doc/bert>` (as well as
|
||||
:doc:`DistilBERT <model_doc/distilbert>` and :doc:`Electra <model_doc/electra>`) and was outlined in
|
||||
`this paper <https://static.googleusercontent.com/media/research.google.com/ja//pubs/archive/37842.pdf>`__. It relies
|
||||
on the same base as BPE, which is to initialize the vocabulary to every character present in the corpus and
|
||||
progressively learn a given number of merge rules, the difference is that it doesn't choose the pair that is the most
|
||||
frequent but the one that will maximize the likelihood on the corpus once merged.
|
||||
|
||||
What does this mean? Well, in the previous example, it means we would only merge 'u' and 'g' if the probability of
|
||||
having 'ug' divided by the probability of having 'u' then 'g' is greater than for any other pair of symbols. It's
|
||||
subtly different from what BPE does in the sense that it evaluates what it "loses" by merging two symbols and makes
|
||||
sure it's `worth it`.
|
||||
|
||||
.. _unigram:
|
||||
|
||||
Unigram
|
||||
=======
|
||||
|
||||
Unigram is a subword tokenization algorithm introduced in `this paper <https://arxiv.org/pdf/1804.10959.pdf>`__.
|
||||
Instead of starting with a group of base symbols and learning merges with some rule, like BPE or WordPiece, it starts
|
||||
from a large vocabulary (for instance, all pretokenized words and the most common substrings) that it will trim down
|
||||
progressively. It's not used directly for any of the pretrained models in the library, but it's used in conjunction
|
||||
with :ref:`SentencePiece <sentencepiece>`.
|
||||
|
||||
More specifically, at a given step, unigram computes a loss from the corpus we have and the current vocabulary, then,
|
||||
for each subword, evaluate how much the loss would augment if the subword was removed from the vocabulary. It then
|
||||
sorts the subwords by this quantity (that represents how worse the loss becomes if the token is removed) and removes
|
||||
all the worst p tokens (for instance p could be 10% or 20%). It then repeats the process until the vocabulary has
|
||||
reached the desired size, always keeping the base characters (to be able to tokenize any word written with them, like
|
||||
BPE or WordPiece).
|
||||
|
||||
Contrary to BPE and WordPiece that work out rules in a certain order that you can then apply in the same order when
|
||||
tokenizing new text, Unigram will have several ways of tokenizing a new text. For instance, if it ends up with the
|
||||
vocabulary
|
||||
|
||||
::
|
||||
|
||||
['b', 'g', 'h', 'n', 'p', 's', 'u', 'ug', 'un', 'hug']
|
||||
|
||||
we had before, it could tokenize "hugs" as ``['hug', 's']``, ``['h', 'ug', 's']`` or ``['h', 'u', 'g', 's']``. So which
|
||||
one choose? On top of saving the vocabulary, the trained tokenizer will save the probability of each token in the
|
||||
training corpus. You can then give a probability to each tokenization (which is the product of the probabilities of the
|
||||
tokens forming it) and pick the most likely one (or if you want to apply some data augmentation, you could sample one
|
||||
of the tokenization according to their probabilities).
|
||||
|
||||
Those probabilities are what are used to define the loss that trains the tokenizer: if our corpus consists of the
|
||||
words :math:`x_{1}, \dots, x_{N}` and if for the word :math:`x_{i}` we note :math:`S(x_{i})` the set of all possible
|
||||
tokenizations of :math:`x_{i}` (with the current vocabulary), then the loss is defined as
|
||||
|
||||
.. math::
|
||||
\mathcal{L} = -\sum_{i=1}^{N} \log \left ( \sum_{x \in S(x_{i})} p(x) \right )
|
||||
|
||||
.. _sentencepiece:
|
||||
|
||||
SentencePiece
|
||||
=============
|
||||
|
||||
All the methods we have been looking at so far required some from of pretrokenization, which has a central problem: not
|
||||
all languages use spaces to separate words. This is a problem :doc:`XLM <model_doc/xlm>` solves by using specific
|
||||
pretokenizers for each of those languages (in this case, Chinese, Japanese and Thai). To solve this problem,
|
||||
SentencePiece (introduced in `this paper <https://arxiv.org/pdf/1808.06226.pdf>`__) treats the input as a raw stream,
|
||||
includes the space in the set of characters to use, then uses BPE or unigram to construct the appropriate vocabulary.
|
||||
|
||||
That's why in the example we saw before using :class:`~transformers.XLNetTokenizer` (which uses SentencePiece), we had
|
||||
some '▁' characters, that represent spaces. Decoding a tokenized text is then super easy: we just have to concatenate
|
||||
all of them together and replace those '▁' by spaces.
|
||||
|
||||
All transformers models in the library that use SentencePiece use it with unigram. Examples of models using it are
|
||||
:doc:`ALBERT <model_doc/albert>`, :doc:`XLNet <model_doc/xlnet>` or the :doc:`Marian framework <model_doc/marian>`.
|
Loading…
Reference in New Issue