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Homework 5: Neural Machine Translation

Due: November 17th, 2017

Imagine it’s 2022 and you are training a German-English neural machine translation (NMT) system. Suddenly, a nuclear bomb is detonated in Los Angeles and caused a massive blackout. As a result, all the neural machine translation codebase in the world is destroyed. Fortunately, you placed your data and the most recent model dump in this Google drive folder, which is preserved during the blackout.

Your challenge is to build a neural machine translation system with the best possible translation quality with these resources. To save your computation resource usage, you can start training by loading the preserved model dump. Because this model is not trained until convergence, you can definitely improve over this model by running a few more training iterations (which is normally called “continued training”) and beat the baseline. (To give you an idea of how large the data is, it’ll take about 10 hours to run the full training on one Tesla K80 GPU of Google Cloud Platform. The model dump you have has been trained for about 6 hours.)

However, to be able to run continued training, you need to restore the NMT codebase by implementing your own neural machine translation model. The model we are looking for you to implement is a attention-based neural machine translation model, which is described by (Bahdanau et al. 2015) and (Luong et al. 2015) as well as (Koehn 2017). You will then run contineud training starting with your model dump and test your final model by translating the test set and submit the translation to the leaderboard submission site (Don’t forget to run post-processing before you submitt – see Setup section). To make your life easier, you don’t have to implement beam search to beat the baseline model. After that, you may earn full credit by implementing at least one more improvement over the baseline attention-based neural machine translation model. Here are several ideas:

  • Implement one of the deep architectures. We don’t expect you to do much, as it’ll take enormous time and resource to train a lot of deep models. For example, if you can train a two-level stacked encoder, that’ll count as your improvement.
  • Implement ensemble of multiple models, as introduced in page 41 of this slide.
  • Adding a coverage model as what we had in SMT decoding.
  • Play with one of the character-based models, like this one or this one.
  • Implement beam search. You may also experiment with multiple beam search strategies.

All the parts of this homework, including baseline and improvements, DO NOT subject to limits on usage of deep learning frameworks (of course, you cannot use a module that implements the seq2seq model for you).

Neural machine translation is a very active resesarch field in recent years and there are new ideas popping up every day, so be creative and don’t be constrained by the list above! If your idea does not fit into the two-week homework span, you can consider making this your final project. Note that some of these ideas may require you to train part or all the model from scratch, so it may take longer than the original continued training.


Pull or check out the starter code. Download the necessary data files from here.

As you have seen in homework 4, you need to preprocess the training data into a list of word indexes. The only difference for this homework is that you need to do this respectively for German and English.

python --train_file --dev_file --test_file --vocab_file model.src.vocab --data_file
python --train_file trn.en --dev_file dev.en --test_file devtest.fake.en --vocab_file model.trg.vocab --data_file hw5.en --charniak

The format of the data is the same as the last homework. The data dump on each side of the parallel corpus contains a tuple (train_data, dev_data, test_data, vocab), each data being a list of torch tensors of size (sent_len,), while the vocabulary is an instance of torchtext.vocab.Vocab.

A starter code ( has been provided for you, which is also pretty similar from what you have seen in the last homework.

Because Google App Engine does not run bash scripts, you’ll also have to run post-processing script yourself before you submit to the leaderboard. Here is out you do it:

./ < [system output] > [post-processed output]

Parameter File Format

The file model.param is a Python dictionary that contains the model dump that’s been trained for 7 epochs on the data provided to you. Although the keys should be self-explanatory, here is a brief description of what they are:

word embeddings

  • encoder.embeddings.emb_luts.0.weight torch.Size([src_vocab_size, src_word_emb_size] = [36616, 300]): the source word embedding
  • decoder.embeddings.emb_luts.0.weight torch.Size([trg_vocab_size, trg_word_emb_size] = [23262, 300]): the target word embedding

forward source encoding:

  • encoder.rnn.weight_ih_l0 torch.Size([4 * encoder_hidden_size, src_word_emb_size] = [2048, 300]): the input connection to the gates of the LSTM, see here for how the weights are arranged
  • encoder.rnn.weight_hh_l0 torch.Size([4 * encoder_hidden_size, encoder_hidden_size] = [2048, 512]): the hidden connection to the gates of the LSTM, see here for how the weights are arranged
  • encoder.rnn.bias_ih_l0 torch.Size([4 * encoder_hidden_size] = [2048]): bias term for the input connections, same arrangement as above
  • encoder.rnn.bias_hh_l0 torch.Size([4 * encoder_hidden_size] = [2048]): bias term for the hidden connections, same arrangement as above

backward source encoding (same thing as above):

  • encoder.rnn.weight_ih_l0_reverse torch.Size([2048, 300])
  • encoder.rnn.weight_hh_l0_reverse torch.Size([2048, 512])
  • encoder.rnn.bias_ih_l0_reverse torch.Size([2048])
  • encoder.rnn.bias_hh_l0_reverse torch.Size([2048])


The decoder is yet another LSTM, as the encoders. However, a slight difference is that the input to the decoder LSTM is the concatenation of context vector and the target side word embedding of the previous output word. The context vector is the output of the attention layer, which you’ll learn about in the subsequent sections.

  • decoder.rnn.layers.0.weight_ih torch.Size([4 * decoder_hidden_size, trg_word_emb_size + context_vector_size] = [4096, 1324])
  • decoder.rnn.layers.0.weight_hh torch.Size([4 * decoder_hidden_size, decoder_hidden_size] = [4096, 1024])
  • decoder.rnn.layers.0.bias_ih torch.Size([4 * decoder_hidden_size] = [4096])
  • decoder.rnn.layers.0.bias_hh torch.Size([4 * decoder_hidden_size] = [4096])


The generator is the mapping from decoder hidden state to vocabulary distrbution. For the baseline model and training, it’s just a affine transformation (use nn.Linear). But if you want to do beam search, expect it to be more complicated.

  • 0.weight torch.Size([trg_vocab_size, decoder_hidden_size] = [23262, 1024])
  • 0.bias torch.Size([trg_vocab_size] = [23262])


The global general attention described in (Luong et al. 2015) was used in the model, which has some extra complication than the attention described in (Bahdanau et al. 2015). Here is the basic idea of it.

You should have known from the lecture that the Bahdanau attention constructs a summary of the source side information by carrying out a weighted sum over the source side encodings (not word embeddings!), with the weight defined by the content of the decoder hidden state \(h_{t-1}\) at time step \(t-1\) and the source side encoding \(h_s\) of word \(s\). The Luong attention, while still maintaining this weighted sum mechanism, carry out an extra transformation following that. Below we will call the weighted sum of the source side the encoding \(\tilde{s_t}\) and the final summary of the source side information \(c_t\), or context vector. Both of these \(t\) are refering to time step \(t\).

Easily enough, here is how \(s_t\) computed.

\[\tilde{s_t} = \sum_{s=0}^{\mid S\mid} a(h_s, h_{t-1}) * h_s\] \[a(h_s, h_{t-1}) = \dfrac{\exp(score(h_s, h_{t-1}))}{\sum_{s=0}^{\mid S\mid} \exp(score(h_s, h_{t-1}))}\]

Note that each \(h_s\) is the concatenation of forward and backward encoding, so its dimension is 2 * encoder_hidden_size = 1024.

This weighted sum \(s_t\) is then combined with previous decoder hidden state \(h_{t-1}\) again to construct the context vector \(c_t\) at timestep \(t\).

\[c_t = tanh(W_o [\tilde{s_t}; h_{t-1}])\]

where the semicolon denotes concatenation. We are using decoder hidden state size 1024 and context vector 1024, so the input of this linear transformation is of dimension 2 * encoder_hidden_size + decoder_hidden_size = 2048 while the output is context_vector_size = 1024. The question now is: how do we compute \(score(h_s, h_{t-1})\)? The global attention calculates it in the following way:

\[score(h_s, h_{t-1}) = h_s^T W_i h_{t-1}\]

Hence, you need the following two weights to implement global attention.

  • decoder.attn.linear_in.weight torch.Size([1024, 1024]): \(W_i\)
  • decoder.attn.linear_out.weight torch.Size([1024, 2048]): \(W_o\)

Cloud Guide (2017)

To avoid further nuclear attacks, your supervisor from LAPD has instructed you to use Google Cloud to run GPU training. Please read through this note so you know how to use the Google Cloud GPU host.

Please keep in mind that you should use your GPU hours wisely since the $50 credit will only give you ~60 hours of GPU usage. Always debug on CPU before running your program on GPU!

Deep Learning Framework Choice Policy (2017)

We understand people may have different preferences for deep learning frameworks, but as new frameworks come out everyday, it is not possible for us as instructors and TAs to cover knowledge of all the frameworks. As of 2017, we will use PyTorch for all the starter code and is only able to help if you are using PyTorch. PyTorch is quickly gaining popularity among NLP/MT/ML research community and is, from our perspective, relatively easy to pick up as a beginner.

If you prefer using another framework, we welcome contribution of your starter code (and you’ll get credit as contributors in a homework that’ll likely to be used in the coming years), but again, we are not able to help if you run into problems with the framework.

Your choice of framework will not affect the grade of your homework.

Ground Rules

  • You can work in independently or in groups of up to three, under these conditions:
    1. You must announce the group publicly on piazza.
    2. You agree that everyone in the group will receive the same grade on the assignment.
    3. You can add people or merge groups at any time before the assignment is due. You cannot drop people from your group once you’ve added them. We encourage collaboration, but we will not adjudicate Rashomon-style stories about who did or did not contribute.
  • You must turn in three things:
    1. Your translation of the whole test set, uploaded to the leaderboard submission site. You can upload new output as often as you like, up until the assignment deadline.

    2. Your code. Send us a URL from which we can get the code and git revision history (a link to a tarball will suffice, but you’re free to send us a github link if you don’t mind making your code public). This is due at the deadline: when you upload your final answer, send us the code. You are free to extend the code we provide or roll your own in whatever langugage you like, but the code should be self-contained, self-documenting, and easy to use.
    3. A clear, mathematical answer for each bullet point with writeup requirements. This needn’t be long, but it should be clear enough that one of your fellow students could re-implement it exactly. If you modified your algorithm or have more than 1 algorithm, explain each modification/algorithm clearly. Give the dev scores for each modification/algorithm, and the test score for your final choice.
  • As most of the storage media is destroyed during blackout, you shouldn’t access any other data than what we provide. You are free to use any code or software you like, but you must write your own NMT model. If you want to use POS tagger, parser, or word embedding builders, that is fine. But any module or program that already implement NMT models for you are off-limit. If you aren’t sure whether something is permitted, ask us. If you want to do system combination, join forces with your classmates.
  • The deadline for the leaderboard is 11-17-2017 at 11:59pm.

*Credits: This assignment was developed by Shuoyang Ding. The idea of blackout was borrowed from the anime short Blade Runner: Blackout 2022.