Assignment 3: Attention-Based Neural Machine Translation

CSC321 Programming Assignment 3
Programming Assignment 3: Attention-Based Neural Machine Translation

Submission: You must submit two files through MarkUs1
: a PDF file containing your writeup,
titled a3-writeup.pdf, and your completed code file models.py. Your writeup must be typeset
using LATEX.
The programming assignments are individual work. See the Course Information handout2
for detailed policies.
You should attempt all questions for this assignment. Most of them can be answered at least partially even if you were unable to finish earlier questions. If you were unable to run the experiments,
please discuss what outcomes you might hypothetically expect from the experiments. If you think
your computational results are incorrect, please say so; that may help you get partial credit.
Introduction
In this assignment, you will train an attention-based neural machine translation model to translate
words from English to Pig-Latin. Along the way, you’ll gain experience with several important
concepts in NMT, including attention and teacher forcing.
Pig Latin
Pig Latin is a simple transformation of English based on the following rules (applied on a per-word
basis):
1. If the first letter of a word is a consonant, then the letter is moved to the end of the word,
and the letters “ay” are added to the end: team → eamtay.
2. If the first letter is a vowel, then the word is left unchanged and the letters “way” are added
to the end: impress → impressway.
3. In addition, some consonant pairs, such as “sh”, are treated as a block and are moved to the
end of the string together: shopping → oppingshay.
To translate a whole sentence from English to Pig-Latin, we simply apply these rules to each word
independently:
i went shopping → iway entway oppingshay
We would like a neural machine translation model to learn the rules of Pig-Latin implicitly,
from (English, Pig-Latin) word pairs. Since the translation to Pig Latin involves moving characters
around in a string, we will use character-level recurrent neural networks for our model.
1
https://markus.teach.cs.toronto.edu/csc321-2018-01
2
http://www.cs.toronto.edu/~rgrosse/courses/csc321_2018/syllabus.pdf
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CSC321 Winter 2018 Programming Assignment 3
Because English and Pig-Latin are so similar in structure, the translation task is almost a copy
task; the model must remember each character in the input, and recall the characters in a specific
order to produce the output. This makes it an ideal task for understanding the capacity of NMT
models.
Data
The data for this task consists of pairs of words {(s
(i)
, t(i)
)}
N
i=1 where the source s
(i)
is an English
word, and the target t
(i)
is its translation in Pig-Latin. The dataset is composed of unique words
from the book “Sense and Sensibility,” by Jane Austen. The vocabulary consists of 29 tokens:
the 26 standard alphabet letters (all lowercase), the dash symbol -, and two special tokens <SOS>
and <EOS> that denote the start and end of a sequence, respectively. 3 The dataset contains 6387
unique (English, Pig-Latin) pairs in total; the first few examples are:
{ (the, ethay), (family, amilyfay), (of, ofway), ... }
In order to simplify the processing of mini-batches of words, the word pairs are grouped based
on the lengths of the source and target. Thus, in each mini-batch the source words are all the same
length, and the target words are all the same length. This simplifies the code, as we don’t have to
worry about batches of variable-length sequences.
Part 1: Encoder-Decoder Models and Capacity [1 mark]
Translation is a sequence-to-sequence problem: in our case, both the input and output are sequences
of characters. A common architecture used for seq-to-seq problems is the encoder-decoder model [2],
composed of two RNNs, as follows:
c a t <EOS> <SOS> a t c a y
a t c a y <EOS>
Encoder Decoder
Training
Figure 1: Training the NMT encoder-decoder architecture.
The encoder RNN compresses the input sequence into a fixed-length vector, represented by
the final hidden state hT . The decoder RNN conditions on this vector to produce the translation,
character by character.
Input characters are passed through an embedding layer before they are fed into the encoder
RNN; in our model, we learn a 29 × 10 embedding matrix, where each of the 29 characters in the
vocabulary is assigned a 10-dimensional embedding. At each time step, the decoder RNN outputs a
vector of unnormalized log probabilities given by a linear transformation of the decoder hidden state.
When these probabilities are normalized, they define a distribution over the vocabulary, indicating
3Note that for the English-to-Pig-Latin task, the input and output sequences share the same vocabulary; this is
not always the case for other translation tasks (i.e., between languages that use different alphabets).
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CSC321 Winter 2018 Programming Assignment 3
the most probable characters for that time step. The model is trained via a cross-entropy loss
between the decoder distribution and ground-truth at each time step.
Conceptual Questions
1. How do you think this architecture will perform on long sequences, and why? Consider the
amount of information the decoder gets to see about the input sequence.
2. In the code folder, you will find a pre-trained model of the above architecture, using a hidden
state of size 10. This model was trained to convergence. The script translate_no_attn.py
uses this pre-trained model to translate words given in the list words. Run this script by
calling:
python translate_no_attn.py
How do the results look, qualitatively? Does the model do better for certain types of words
than others? Add a few of your own words to the words list at the top of the script, and run
it again. Which failure modes can you identify?
Part 2: Teacher-Forcing [1 mark]
c a t <EOS> <SOS>
a t c a y <EOS>
Encoder Decoder
Generation
Figure 2: Generating text with the NMT encoder-decoder architecture.
The decoder produces a distribution over the output vocabulary conditioned on the previous
hidden state and the output token in the previous timestep. A common practice used to train
NMT models is to feed in the ground-truth token from the previous time step to condition the
decoder output in the current step, as shown in Figure 1. At test time, we don’t have access to the
ground-truth output sequence, so the decoder must condition its output on the token it generated
in the previous time step, as shown in Figure 2.
Questions
1. What problem may arise when training with teacher forcing? Consider the differences that
arise when we switch from training to testing.
2. Can you think of any way to address this issue? Read the abstract and introduction of the
paper “Scheduled sampling for sequence prediction with recurrent neural networks” [1], and
answer this question in your own words.
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CSC321 Winter 2018 Programming Assignment 3
Teacher-Forcing Ratio (Optional)
In the starter code, teacher-forcing is used 50% of the time, and the model’s own predictions
are used 50% of the time when training (see [1]). If you want to observe the effects of using
teacher-forcing more or less of the time, you can provide your own teacher-forcing ratio to train the
model; for example, python attention_nmt.py --teacher_forcing_ratio=1 trains purely with
teacher-forcing. This is optional, and not required for this assignment.
Part 3: Gated Recurrent Unit (GRU) [2 marks]
Throughout the rest of the assignment, you will implement an attention-based neural machine
translation model, and finally train the model and examine the results.
1. The forward pass of a Gated Recurrent Unit is defined by the following equations:
rt = σ(Wirxt + Whrht−1 + br) (1)
zt = σ(Wizxt + Whzht−1 + bz) (2)
gt = tanh(Winxt + rt   (Whnht−1 + bg)) (3)
ht = (1 − z)   gt + z   ht−1 (4)
Although PyTorch has a GRU built in (nn.GRUCell), we’ll implement our own GRU cell from
scratch, to better understand how it works. Fill in the __init__ and forward methods of
the MyGRUCell class in models.py, to implement the above equations. A template has been
provided for the forward method.
Part 4: Implementing Attention [4 marks]
Attention allows a model to look back over the input sequence, and focus on relevant input tokens
when producing the corresponding output tokens. For our simple task, attention can help the
model remember tokens from the input, e.g., focusing on the input letter c to produce the output
letter c.
The hidden states produced by the encoder while reading the input sequence, h
enc
1
, . . . , henc
T
can
be viewed as annotations of the input; each encoder hidden state h
enc
i
captures information about
the i
th input token, along with some contextual information. At each time step, an attention-based
decoder computes a weighting over the annotations, where the weight given to each one indicates
its relevance in determining the current output token.
In particular, at time step t, the decoder computes an attention weight α
(t)
i
for each of the
encoder hidden states h
enc
i
. The weights are defined such that 0 ≤ α
(t)
i ≤ 1 and P
i α
(t)
i = 1. α
(t)
i
is
a function of an encoder hidden state and the previous decoder hidden state, f(h
dec
t−1
, henc
i
), where
i ranges over the length of the input sequence. One possible function f is the dot product, which
measures the similarity between the two hidden states.
1. For our model, we will learn the function f, parameterized as a two-layer fully-connected
network with a ReLU activation. This network produces unnormalized weights ˜α
(t)
i
as:
α˜
(t)
i = f(h
dec
t−1
, henc
i
) = W2(max(0, W1[h
dec
t−1
; h
enc
i
] + b1)) + b2
Here, the notation [h
dec
t−1
; h
enc
i
] denotes the concatenation of vectors h
dec
t−1
and h
enc
i
. Because
the attention weights must be normalized, we need to apply the softmax function over the
output of the two-layer network: α
(t)
i = softmax(˜α
(t)
)i
.
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CSC321 Winter 2018 Programming Assignment 3
Implement this two-layer attention mechanism. Fill in the __init__ and forward
methods of the Attention class in models.py. Use the PyTorch nn.Sequential class to
define the attention network, and use the self.softmax function in the forward pass of the
Attention class to normalize the weights.
...
+
Decoder Hidden States Encoder Hidden States
batch_size
batch_size
seq_len
hidden_size hidden_size
batch_size
seq_len
1
Attention Weights
Figure 3: Dimensions of the input and output tensors of the Attention module.
For the forward pass, you will need to do some reshaping of tensors. You are given a batch of
decoder hidden states for time t − 1, which has dimension batch_size x hidden_size, and
a batch of encoder hidden states (annotations) for each timestep in the input sequence, which
has dimension batch_size x seq_len x hidden_size. The goal is to compute the function
f(h
dec
t−1
, henc
i
) for each decoder hidden state in the batch and all corresponding encoder hidden
states h
enc
i
, where i ranges over seq_len different values. You must do this in a vectorized
fashion. Since f(h
dec
t−1
, henc
i
) is a scalar, the resulting tensor of attention weights should have
dimension batch_size x seq_len x 1. The input and output dimensions of the Attention
module are visualized in Figure 3.
Depending on your implementation, you will need one or more of these functions (click to
jump to the PyTorch documentation):
• squeeze
• unsqueeze
• expand as
• cat
• view
The self.attention_network module takes as input a 2-dimensional tensor; you will need to
view a 3D tensor as a 2D tensor to pass it through the attention network, and then view it as
a 3D tensor again. We have provided a template for the forward method of the Attention
class. You are free to use the template, or code it from scratch, as long as the output is
correct.
2. Once we have the attention weights, a context vector ct
is computed as a linear combination
of the encoder hidden states, with coefficients given by the weights:
ct =
X
T
i=1
α
(t)
i
h
enc
i
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CSC321 Winter 2018 Programming Assignment 3
This context vector is concatenated with the input vector and passed into the decoder GRU
cell at each time step, as shown in Figure 4.
...
+
h1
enc
α1 αT
Figure 4: Computing a context vector with attention.
Fill in the forward method of the AttentionDecoder class, to implement the interface shown
in Figure 4. You will need to:
(a) Compute the attention weights using self.attention_network
(b) Multiply these weights by the corresponding encoder hidden states and sum them to
form the context vector.
(c) Concatenate the context vector with the current decoder input.
(d) Feed the concatenation to the decoder GRU cell to obtain the new hidden state.
(e) Compute the output using self.out.
3. Train the model with attention by running the following command:
python attention_nmt.py
By default, the script runs for 100 epochs, which should be enough to get good results; this
takes approximately 24 minutes on the teaching lab machines. If necessary, you can train for
fewer epochs by running python attention_nmt.py --nepochs=50, or you can exit training
early with Ctrl-C.
At the end of each epoch, the script prints training and validation losses, and the Pig-Latin
translation of a fixed sentence, “the air conditioning is working”, so that you can see how
the model improves qualitatively over time. The fixed sentence is stored in the variable
TEST_SENTENCE, at the top of attention_nmt.py. You can change this variable to see how
translation improves for your own sentence!
The script also saves several items to the directory checkpoints/h10-bs16:
• The best encoder and decoder model paramters, based on the validation loss.
• A plot of the training and validation losses.
• Attention maps generated during training for a fixed word, given by the variable TEST_WORD_ATTN
in attention_nmt.py. These maps allow you to see how the attention improves over
the course of training.
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CSC321 Winter 2018 Programming Assignment 3
Part 5: Attention Visualizations [2 marks]
One of the benefits of using attention is that it allows us to gain insight into the inner workings
of the model. By visualizing the attention weights generated for the input tokens in each decoder
step, we can see where the model focuses while producing each output token. In this part of the
assignment, you will visualize the attention learned by your model, and try to find interesting
success and failure modes that illustrate its behaviour.
The script visualize_attention.py loads a pre-trained model and uses it to translate a given
set of words: it prints the translations and saves heatmaps to show how attention is used at each
step. To call this script, you need to pass in the path to a checkpoint folder, as follows:
python visualize_attention.py --load checkpoints/h10-bs16
1. The visualize_attention.py script produces visualizations for the strings in the words list,
found at the top of the script. The visualizations are saved as PDF files in the same directory
as the loaded model checkpoint, so they will be in checkpoints/h10-bs16. Add your own
strings to words list in visualize_attention.py and run the script as shown above. Since
the model operates at the character-level, the input doesn’t even have to be a real word in the
dictionary. You can be creative! After running the script, you should examine the generated
attention maps. Try to find failure cases, and hypothesize about why they occur. Some
interesting classes of words you may want to try are:
• Words that begin with a single consonant (e.g., cake).
• Words that begin with two or more consonants (e.g., drink).
• Words that have unusual/rare letter combinations (e.g., aardvark).
• Compound words consisting of two words separated by a dash (e.g., well-mannered).
These are the hardest class of words present in the training data, because they are
long, and because the rules of Pig-Latin dictate that each part of the word (e.g., well
and mannered) must be translated separately, and stuck back together with a dash:
ellway-annerdmay.
• Made-up words or toy examples to show a particular behaviour.
Include attention maps for both success and failure cases in your writeup, along
with your hypothesis about why the model succeeds or fails.
What you need to submit
• One code file: models.py.
• A PDF document titled a3-writeup.pdf containing your answers to the conceptual questions,
and the attention visualizations, with explanations.
References
[1] Samy Bengio, Oriol Vinyals, Navdeep Jaitly, and Noam Shazeer. Scheduled sampling for sequence prediction with recurrent neural networks. In Advances in Neural Information Processing
Systems, pages 1171–1179, 2015.
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CSC321 Winter 2018 Programming Assignment 3
[2] Ilya Sutskever, Oriol Vinyals, and Quoc V Le. Sequence to sequence learning with neural
networks. In Advances in neural information processing systems, pages 3104–3112, 2014.
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