.. _chapter_language_model:
Language Models and Data Sets
=============================
In :numref:`chapter_text_preprocessing`, we see how to map text data
into tokens, and these tokens can be viewed as a time series of discrete
observations. Assuming the tokens in a text of length :math:`T` are in
turn :math:`x_1, x_2, \ldots, x_T`, then, in the discrete time series,
:math:`x_t`\ (:math:`1 \leq t \leq T`) can be considered as the output
or label of time step :math:`t`. Given such a sequence, the goal of a
language model is to estimate the probability
.. math:: p(x_1,x_2, \ldots, x_T).
Language models are incredibly useful. For instance, an ideal language
model would be able to generate natural text just on its own, simply by
drawing one word at a time :math:`w_t \sim p(w_t|w_{t-1}, \ldots w_1)`.
Quite unlike the monkey using a typewriter, all text emerging from such
a model would pass as natural language, e.g. English text. Furthermore,
it would be sufficient for generating a meaningful dialog, simply by
conditioning the text on previous dialog fragments. Clearly we are still
very far from designing such a system, since it would need to
*understand* the text rather than just generate grammatically sensible
content.
Nonetheless language models are of great service even in their limited
form. For instance, the phrases *‘to recognize speech’* and *‘to wreck a
nice beach’* sound very similar. This can cause ambiguity in speech
recognition, ambiguity that is easily resolved through a language model
which rejects the second translation as outlandish. Likewise, in a
document summarization algorithm it’s worth while knowing that *‘dog
bites man’* is much more frequent than *‘man bites dog’*, or that
*‘let’s eat grandma’* is a rather disturbing statement, whereas *‘let’s
eat, grandma’* is much more benign.
Estimating a language model
---------------------------
The obvious question is how we should model a document, or even a
sequence of words. We can take recourse to the analysis we applied to
sequence models in the previous section. Let’s start by applying basic
probability rules:
.. math:: p(w_1, w_2, \ldots, w_T) = \prod_{t=1}^T p(w_t | w_1, \ldots, w_{t-1}).
For example, the probability of a text sequence containing four tokens
consisting of words and punctuation would be given as:
.. math:: p(\mathrm{Statistics}, \mathrm{is}, \mathrm{fun}, \mathrm{.}) = p(\mathrm{Statistics}) p(\mathrm{is} | \mathrm{Statistics}) p(\mathrm{fun} | \mathrm{Statistics}, \mathrm{is}) p(\mathrm{.} | \mathrm{Statistics}, \mathrm{is}, \mathrm{fun}).
In order to compute the language model, we need to calculate the
probability of words and the conditional probability of a word given the
previous few words, i.e. language model parameters. Here, we assume that
the training data set is a large text corpus, such as all Wikipedia
entries, Project Gutenberg, or all text posted online on the web. The
probability of words can be calculated from the relative word frequency
of a given word in the training data set.
For example, :math:`p(\mathrm{Statistics})` can be calculated as the
probability of any sentence starting with the word ‘statistics’. A
slightly less accurate approach would be to count all occurrences of the
word ‘statistics’ and divide it by the total number of words in the
corpus. This works fairly well, particularly for frequent words. Moving
on, we could attempt to estimate
.. math:: \hat{p}(\mathrm{is}|\mathrm{Statistics}) = \frac{n(\mathrm{Statistics~is})}{n(\mathrm{Statistics})}.
Here :math:`n(w)` and :math:`n(w, w')` are the number of occurrences of
singletons and pairs of words respectively. Unfortunately, estimating
the probability of a word pair is somewhat more difficult, since the
occurrences of *‘Statistics is’* are a lot less frequent. In particular,
for some unusual word combinations it may be tricky to find enough
occurrences to get accurate estimates. Things take a turn for the worse
for 3 word combinations and beyond. There will be many plausible 3-word
combinations that we likely won’t see in our dataset. Unless we provide
some solution to give such word combinations nonzero weight we will not
be able to use these as a language model. If the dataset is small or if
the words are very rare, we might not find even a single one of them.
A common strategy is to perform some form of Laplace smoothing. We
already encountered this in our discussion of naive bayes in
:numref:`chapter_naive_bayes` where the solution was to add a small
constant to all counts. This helps with singletons, e.g. via
.. math::
\begin{aligned}
\hat{p}(w) & = \frac{n(w) + \epsilon_1/m}{n + \epsilon_1} \\
\hat{p}(w'|w) & = \frac{n(w,w') + \epsilon_2 \hat{p}(w')}{n(w) + \epsilon_2} \\
\hat{p}(w''|w',w) & = \frac{n(w,w',w'') + \epsilon_3 \hat{p}(w',w'')}{n(w,w') + \epsilon_3}
\end{aligned}
Here the coefficients :math:`\epsilon_i > 0` determine how much we use
the estimate for a shorter sequence as a fill-in for longer ones.
Moreover, :math:`m` is the total number of words we encounter. The above
is a rather primitive variant of what is Kneser-Ney smoothing and
Bayesian Nonparametrics can accomplish. See e.g. the Sequence Memoizer
of Wood et al., 2012 for more details of how to accomplish this.
Unfortunately, models like this get unwieldy rather quickly: first off,
we need to store all counts and secondly, this entirely ignores the
meaning of the words. For instance, *‘cat’* and *‘feline’* should occur
in related contexts. Deep learning based language models are well suited
to take this into account. This, it is quite difficult to adjust such
models to additional context. Lastly, long word sequences are almost
certain to be novel, hence a model that simply counts the frequency of
previously seen word sequences is bound to perform poorly there.
Markov Models and :math:`n`-grams
---------------------------------
Before we discuss solutions involving deep learning we need some more
terminology and concepts. Recall our discussion of Markov Models in the
previous section. Let’s apply this to language modeling. A distribution
over sequences satisfies the Markov property of first order if
:math:`p(w_{t+1}|w_t, \ldots w_1) = p(w_{t+1}|w_t)`. Higher orders
correspond to longer dependencies. This leads to a number of
approximations that we could apply to model a sequence:
.. math::
\begin{aligned}
p(w_1, w_2, w_3, w_4) &= p(w_1) p(w_2) p(w_3) p(w_4)\\
p(w_1, w_2, w_3, w_4) &= p(w_1) p(w_2 | w_1) p(w_3 | w_2) p(w_4 | w_3)\\
p(w_1, w_2, w_3, w_4) &= p(w_1) p(w_2 | w_1) p(w_3 | w_1, w_2) p(w_4 | w_2, w_3)
\end{aligned}
Since they involve one, two or three terms, these are typically referred
to as unigram, bigram and trigram models. In the following we will learn
how to design better models.
Natural Language Statistics
---------------------------
Let’s see how this works on real data. We construct a vocabulary based
on the time machine data similar to
:numref:`chapter_text_preprocessing` and print the top words
.. code:: python
import d2l
from mxnet import nd
import random
tokens = d2l.tokenize(d2l.read_time_machine())
vocab = d2l.Vocab(tokens)
print(vocab.token_freqs[:10])
.. parsed-literal::
:class: output
[('the', 2261), ('', 1282), ('i', 1267), ('and', 1245), ('of', 1155), ('a', 816), ('to', 695), ('was', 552), ('in', 541), ('that', 443)]
As we can see, the most popular words are actually quite boring to look
at. They are often referred to as `stop
words `__ and thus filtered
out. That said, they still carry meaning and we will use them
nonetheless. However, one thing that is quite clear is that the word
frequency decays rather rapidly. The 10th word is less than :math:`1/5`
as common as the most popular one. To get a better idea we plot the
graph of word frequencies.
.. code:: python
freqs = [freq for token, freq in vocab.token_freqs]
d2l.plot(freqs, xlabel='token', ylabel='frequency',
xscale='log', yscale='log')
.. image:: output_lang-model_cb0c0c_3_0.svg
We’re on to something quite fundamental here - the word frequencies
decay rapidly in a well defined way. After dealing with the first four
words as exceptions (‘the’, ‘i’, ‘and’, ‘of’), all remaining words
follow a straight line on a log-log plot. This means that words satisfy
`Zipf’s law `__ which states
that the item frequency is given by
.. math::
n(x) \propto (x + c)^{-\alpha} \text{ and hence }
\log n(x) = -\alpha \log (x+c) + \mathrm{const.}
This should already give us pause if we want to model words by count
statistics and smoothing. After all, we will significantly overestimate
the frequency of the tail, aka the infrequent words. But what about word
pairs (and trigrams and beyond)? Let’s see.
.. code:: python
bigram_tokens = [[pair for pair in zip(line[:-1], line[1:])] for line in tokens]
bigram_vocab = d2l.Vocab(bigram_tokens)
print(bigram_vocab.token_freqs[:10])
.. parsed-literal::
:class: output
[(('of', 'the'), 297), (('in', 'the'), 161), (('i', 'had'), 126), (('and', 'the'), 104), (('i', 'was'), 104), (('the', 'time'), 97), (('it', 'was'), 94), (('to', 'the'), 81), (('as', 'i'), 75), (('of', 'a'), 69)]
Two things are notable. Out of the 10 most frequent word pairs, 9 are
composed of stop words and only one is relevant to the actual book -
‘the time’. Let’s see whether the bigram frequencies behave in the same
manner as the unigram frequencies.
.. code:: python
trigram_tokens = [[triple for triple in zip(line[:-2], line[1:-1], line[2:])]
for line in tokens]
trigram_vocab = d2l.Vocab(trigram_tokens)
print(trigram_vocab.token_freqs[:10])
.. parsed-literal::
:class: output
[(('the', 'time', 'traveller'), 53), (('the', 'time', 'machine'), 24), (('the', 'medical', 'man'), 22), (('it', 'seemed', 'to'), 14), (('it', 'was', 'a'), 14), (('i', 'began', 'to'), 13), (('i', 'did', 'not'), 13), (('i', 'saw', 'the'), 13), (('here', 'and', 'there'), 12), (('i', 'could', 'see'), 12)]
Last, let’s visualize the token frequencies among these three gram
models.
.. code:: python
bigram_freqs = [freq for token, freq in bigram_vocab.token_freqs]
trigram_freqs = [freq for token, freq in trigram_vocab.token_freqs]
d2l.plot([freqs, bigram_freqs, trigram_freqs], xlabel='token',
ylabel='frequency', xscale='log', yscale='log',
legend=['unigram', 'bigram', 'trigram'])
.. image:: output_lang-model_cb0c0c_9_0.svg
The graph is quite exciting for a number of reasons. Firstly, beyond
words, also sequences of words appear to be following Zipf’s law, albeit
with a lower exponent, depending on sequence length. Secondly, the
number of distinct n-grams is not that large. This gives us hope that
there is quite a lot of structure in language. Third, *many* n-grams
occur very rarely, which makes Laplace smoothing rather unsuitable for
language modeling. Instead, we will use deep learning based models.
Training Data Preparation
-------------------------
Before introducing the model, let’s assume we will use a neural network
to train a language model. Now the question is how to read mini-batches
of examples and labels at random. Since sequence data is by its very
nature sequential, we need to address the issue of processing it. We did
so in a rather ad-hoc manner when we introduced in
:numref:`chapter_sequence`. Let’s formalize this a bit.
In :numref:`fig_timemachine_5gram`, we visualized several possible
ways to obtain 5-grams in a sentence, here a token is a character. Note
that we have quite some freedom since we could pick an arbitrary offset.
.. _fig_timemachine_5gram:
.. figure:: ../img/timemachine-5gram.svg
Different offsets lead to different subsequences when splitting up
text.
In fact, any one of these offsets is fine. Hence, which one should we
pick? In fact, all of them are equally good. But if we pick all offsets
we end up with rather redundant data due to overlap, particularly if the
sequences are long. Picking just a random set of initial positions is no
good either since it does not guarantee uniform coverage of the array.
For instance, if we pick :math:`n` elements at random out of a set of
:math:`n` with random replacement, the probability for a particular
element not being picked is :math:`(1-1/n)^n \to e^{-1}`. This means
that we cannot expect uniform coverage this way. Even randomly permuting
a set of all offsets does not offer good guarantees. Instead we can use
a simple trick to get both *coverage* and *randomness*: use a random
offset, after which one uses the terms sequentially. We describe how to
accomplish this for both random sampling and sequential partitioning
strategies below.
Random Sampling
~~~~~~~~~~~~~~~
The following code randomly generates a minibatch from the data each
time. Here, the batch size ``batch_size`` indicates to the number of
examples in each mini-batch and ``num_steps`` is the length of the
sequence (or time steps if we have a time series) included in each
example. In random sampling, each example is a sequence arbitrarily
captured on the original sequence. The positions of two adjacent random
mini-batches on the original sequence are not necessarily adjacent. The
target is to predict the next character based on what we’ve seen so far,
hence the labels are the original sequence, shifted by one character.
.. code:: python
# Save to the d2l package.
def seq_data_iter_random(corpus, batch_size, num_steps):
# Offset the iterator over the data for uniform starts
corpus = corpus[random.randint(0, num_steps):]
# Subtract 1 extra since we need to account for label
num_examples = ((len(corpus) - 1) // num_steps)
example_indices = list(range(0, num_examples * num_steps, num_steps))
random.shuffle(example_indices)
# This returns a sequence of the length num_steps starting from pos
data = lambda pos: corpus[pos: pos + num_steps]
# Discard half empty batches
num_batches = num_examples // batch_size
for i in range(0, batch_size * num_batches, batch_size):
# Batch_size indicates the random examples read each time
batch_indices = example_indices[i:(i+batch_size)]
X = [data(j) for j in batch_indices]
Y = [data(j + 1) for j in batch_indices]
yield nd.array(X), nd.array(Y)
Let us generate an artificial sequence from 0 to 30. We assume that the
batch size and numbers of time steps are 2 and 5 respectively. This
means that depending on the offset we can generate between 4 and 5
:math:`(x,y)` pairs. With a minibatch size of 2 we only get 2
minibatches.
.. code:: python
my_seq = list(range(30))
for X, Y in seq_data_iter_random(my_seq, batch_size=2, num_steps=6):
print('X: ', X, '\nY:', Y)
.. parsed-literal::
:class: output
X:
[[14. 15. 16. 17. 18. 19.]
[ 8. 9. 10. 11. 12. 13.]]
Y:
[[15. 16. 17. 18. 19. 20.]
[ 9. 10. 11. 12. 13. 14.]]
X:
[[ 2. 3. 4. 5. 6. 7.]
[20. 21. 22. 23. 24. 25.]]
Y:
[[ 3. 4. 5. 6. 7. 8.]
[21. 22. 23. 24. 25. 26.]]
Sequential partitioning
~~~~~~~~~~~~~~~~~~~~~~~
In addition to random sampling of the original sequence, we can also
make the positions of two adjacent random mini-batches adjacent in the
original sequence.
.. code:: python
# Save to the d2l package.
def seq_data_iter_consecutive(corpus, batch_size, num_steps):
# Offset for the iterator over the data for uniform starts
offset = random.randint(0, num_steps)
# Slice out data - ignore num_steps and just wrap around
num_indices = ((len(corpus) - offset - 1) // batch_size) * batch_size
Xs = nd.array(corpus[offset:offset+num_indices])
Ys = nd.array(corpus[offset+1:offset+1+num_indices])
Xs, Ys = Xs.reshape((batch_size, -1)), Ys.reshape((batch_size, -1))
num_batches = Xs.shape[1] // num_steps
for i in range(0, num_batches * num_steps, num_steps):
X = Xs[:,i:(i+num_steps)]
Y = Ys[:,i:(i+num_steps)]
yield X, Y
Using the same settings, print input ``X`` and label ``Y`` for each
mini-batch of examples read by random sampling. The positions of two
adjacent random mini-batches on the original sequence are adjacent.
.. code:: python
for X, Y in seq_data_iter_consecutive(my_seq, batch_size=2, num_steps=6):
print('X: ', X, '\nY:', Y)
.. parsed-literal::
:class: output
X:
[[ 1. 2. 3. 4. 5. 6.]
[15. 16. 17. 18. 19. 20.]]
Y:
[[ 2. 3. 4. 5. 6. 7.]
[16. 17. 18. 19. 20. 21.]]
X:
[[ 7. 8. 9. 10. 11. 12.]
[21. 22. 23. 24. 25. 26.]]
Y:
[[ 8. 9. 10. 11. 12. 13.]
[22. 23. 24. 25. 26. 27.]]
Now we wrap the above two sampling functions to a class so that we can
use it as a normal Gluon data iterator later.
.. code:: python
# Save to the d2l package.
class SeqDataLoader(object):
"""A iterator to load sequence data"""
def __init__(self, batch_size, num_steps, use_random_iter, max_tokens):
if use_random_iter:
data_iter_fn = d2l.seq_data_iter_random
else:
data_iter_fn = d2l.seq_data_iter_consecutive
self.corpus, self.vocab = d2l.load_corpus_time_machine(max_tokens)
self.get_iter = lambda: data_iter_fn(self.corpus, batch_size, num_steps)
def __iter__(self):
return self.get_iter()
Lastly, we define a function ``load_data_time_machine`` that returns
both the data iterator and the vocabulary, so we can use it similarly as
other functions with ``load_data`` prefix.
.. code:: python
# Save to the d2l package.
def load_data_time_machine(batch_size, num_steps, use_random_iter=False,
max_tokens=10000):
data_iter = SeqDataLoader(
batch_size, num_steps, use_random_iter, max_tokens)
return data_iter, data_iter.vocab
Summary
-------
- Language models are an important technology for natural language
processing.
- :math:`n`-grams provide a convenient model for dealing with long
sequences by truncating the dependence.
- Long sequences suffer from the problem that they occur very rarely or
never.
- Zipf’s law governs the word distribution for both unigrams and
n-grams.
- There’s a lot of structure but not enough frequency to deal with
infrequent word combinations efficiently via smoothing.
- The main choices for sequence partitioning are whether we pick
consecutive or random sequences.
- Given the overall document length, it is usually acceptable to be
slightly wasteful with the documents and discard half-empty
minibatches.
Exercises
---------
1. Suppose there are 100,000 words in the training data set. How many
word frequencies and multi-word adjacent frequencies does a four-gram
need to store?
2. Review the smoothed probability estimates. Why are they not accurate?
Hint - we are dealing with a contiguous sequence rather than
singletons.
3. How would you model a dialogue?
4. Estimate the exponent of Zipf’s law for unigrams, bigrams and
trigrams.
5. Which other other mini-batch data sampling methods can you think of?
6. Why is it a good idea to have a random offset?
- Does it really lead to a perfectly uniform distribution over the
sequences on the document?
- What would you have to do to make things even more uniform?
7. If we want a sequence example to be a complete sentence, what kinds
of problems does this introduce in mini-batch sampling? Why would we
want to do this anyway?
Scan the QR Code to `Discuss `__
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|image0|
.. |image0| image:: ../img/qr_lang-model.svg