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A novel approach to neural machine translation (facebook.com)
283 points by snippyhollow on May 9, 2017 | hide | past | web | favorite | 44 comments

I'm relatively novice to machine learning but here's my best attempt to summarize what's going on in layman's terms. Please correct me if I'm wrong.

- Encode the words in the source (aka embedding, section 3.1)

- Feed every run of k words into a convolutional layer producing an output, repeat this process 6 layers deep (section 3.2).

- Decide on which input word is most important for the "current" output word (aka attention, section 3.3).

- The most important word is decoded into the target language (section 3.1 again).

You repeat this process with every word as the "current" word. The critical insight of using this mechanism over an RNN is that you can do this repetition in parallel because each "current" word does not depend on any of the previous ones.

Am I on the right track?

Yes, that's pretty accurate. Step 3 (attention) is repeated multiple times, i.e. for each layer in the decoder. With each additional layer, you incorporate more of the previously translated text as well as information about which parts of the source sentence representation were used to generate it. The independence of the current word from the previous words applies to the training phrase as a complete reference translation is provided and the model is trained to predict single next words only. This kind of computation would be very inefficient with an RNN: it would have to run over each word in every layer sequentially which prohibits efficient batching.

When generating a translation for a new sentence, the model uses classic beam search where the decoder is evaluated on a word-by-word basis. It's still pretty fast since the source-side network is highly parallelizable and running the decoder for a single word is relatively cheap.

I really like that Facebook open sources both code and model along with the paper. Most companies don't: e.g. Google, deepmind, Baidu.

Google has released some framework for translation: https://github.com/google/seq2seq/

DeepMind has also released some framework which also has all the building blocks for translation: https://github.com/deepmind/sonnet

Sure but these don't address parent's statement: they don't release code with research. These both came years after the original seq2seq paper.

Google's Seq2Seq came 3-4 months after the NMT paper.

There is a github link in the article with both things: https://github.com/facebookresearch/fairseq

Google and Deepmind released a lot of stuff, I don't feel I have the right to complain about it.

I feel the right to complain anytime Google, Baidu, or Deepmind want to publish results from their models in peer-reviewed forums without offering the models for public scrutiny. If they want to keep the models internal, that's fine, and if they want to be taken seriously in academia that is also fine, but they can't have it both ways.

Ah, yes, Google are not taken seriously in academia because they are not releasing the code source to their models. Google is usually the largest contributor to most ML conferences (NIPS, ICML, etc.)

Most papers are not about implementation and more about the concepts or proofs. They are rather straightforward to reimplement, and I don't think anybody is accusing them of faking their results.

> Google is usually the largest contributor to most ML conferences (NIPS, ICML, etc.)

That is because of their overwhelming influence, not the quality of their publications.

> They are rather straightforward to reimplement

Le and Mikolov's "Distributed Representations of Sentences and Documents", frequently cited as the original example of "doc2vec", could not be reproduced by Mikolov himself. [1]

> and I don't think anybody is accusing them of faking their results.

They sure aren't. That, too, is because of their overwhelming influence. You have to say very nicely that their results are wrong.

For example, here's an IBM research paper that leads and concludes with "we reimplemented doc2vec and made it work well", and whispers "but not as well as Le said". [2]

[1] https://stats.stackexchange.com/questions/123562/has-the-rep...

[2] https://arxiv.org/pdf/1607.05368.pdf

I've been working on doc2vec stuff recently.

The statement Le and Mikolov's "Distributed Representations of Sentences and Documents", frequently cited as the original example of "doc2vec", could not be reproduced by Mikolov himself. is an overstatement - there was only one part that couldn't be completely reproduced.

It's true that Quoc Le's results on the dmpv version of doc2vec have been hard to reproduce. However, the very stackexchange link you cite above points out that it can be reproduced by not shuffling the data. It's likely that this was an oversight.

However - and it's an important thing - the reason this example gets some attention is because doc2vec is a very strong model even in dbow form.

here's an IBM research paper that leads and concludes with "we reimplemented doc2vec and made it work well"

No, they took the Gensim doc2vec implementation and experimented with parameters on different datasets[1].

Also, Mikolov's Word2Vec work was even more important than doc2vec and was fully reproducible and was released with code and trained models, while at Google.

[1] https://github.com/jhlau/doc2vec

> They are rather straightforward to reimplement

Not really. Very often you will find the crucial details that make it work missing. Not sure if things have vastly improved over past 5-6 years.

In academia almost anyone releases the source code of the experiments (at least it was this way 5 years ago, and things doesn't seem to have changed). If it is not true, probe me right.

For example. NIPS proceedings (like hundred of papers): https://papers.nips.cc/book/advances-in-neural-information-p... source code available (around 25, 2 of them in google github repos): https://www.reddit.com/r/MachineLearning/comments/5hwqeb/pro...

> I feel the right to complain anytime

Most people on the Internet do.

Which one do the others release?

Code, usually. Google releases very few models.

> Facebook's mission of making the world more open

That's a rather strong statement, for a company that has become one of the world's most complained-about black boxes.

But yes, they have done a lot of good in the computer science space.

> Facebook's mission of making the world more open

Like many big companies, they want to commoditize their products' complements.

"Smart companies try to commoditize their products' complements." https://www.joelonsoftware.com/2002/06/12/strategy-letter-v/

And they innovated in this space with the "facebook patent grant" -- they give away free stuff, with a patent grant that disappears as soon as you sue them.

And they're better at marketing than many - heard of the amazing new zlib replacement, Zstd? It's better in every way except one - unlike zlib (unconditionally patent free), it is only patent free as long as you don't sue Facebook. But almost no one is aware of that.

Where is the complementary relationship? Is it with the facebook.com website and their open-source AI product? More AI means more people will use Facebook.com?

Or is the comment only tangential to OP?

One logical continuation of adding more attention steps is to make decision of how many attention steps to take determined by the network ala "Adaptive Computation Time for Recurrent Neural Networks", are you planning to go in that direction?

One of my students tried something along these lines for Natural Language Inference (NLI) last year. [1] The results where not conclusive, but perhaps Machine Translation is a better target? My reason for believing this is that the specific dataset for NLI most likely does not require multiple steps of inference for most cases (you can get away with simple token overlap), while the decoder in MT does so since it is constrained to output a single token at each step.

[1]: https://arxiv.org/abs/1610.07647

As far I understood it, Facebook put lots of research into optimizing a certain type of neural network (CNN), while everyone else is using another type called RNN. Up until now, CNN was faster but less accurate. However FB has progressed CNN to the point where it can compete in accuracy, particularly in speech recognition. And most importantly, they are releasing the source code and papers. Does that sound right?

Can anyone else give us an ELI5?

I'll give it a shot.

Traditional Neural Networks worked like this: You have k inputs to a layer, and j outputs, so you have O(k * j) parameters, effectively multiplying the inputs by the parameter to get the outputs. And if you have lots of inputs to each layer, and lots of layers, you have a lot of parameters. Too many parameters = overfitting to your training data pretty quickly. But you want big networks, ideally, to get super accuracy. So the question is how to reduce the number of parameters while still having the same 'power' in the network.

CNNs (Convolutional Neural Networks) solve this problem by tying weights together. Instead of multiplying every input by every output, you build a small set of functions at each layer with a small number of parameters in each, and multiple nearby groups of inputs together. Images are the best way to describe this: a function will take as inputs small (3x3 or 5x5) groups of pixels in the image, and output a single result. But they apply the same function all over the image. Picture a little 5x5 box moving around the image, and running a function at each stop.

This has given some pretty incredible results in the image-recognition problem space, and they're super simple to train.

Another approach, Recurrent Neural networks (RNNs) turns the model around in a different way. Instead of having a long list of inputs that all come at once, it takes each input one at a time (or maybe a group at a time, same idea) and runs the neural-network machinery to build up to a single answer. So you might feed it one word at a time of input in English, and after a few words, it starts outputting one word at a time in French until the inputs run out and the output says its the end of the sentence.

What Facebook is doing is applying CNNs to text-sequence and translation problems. It seems to me that what they have here is kind of a RNN-CNN hybrid.

Caveats: I'm an idiot! I just read a lot and play around with ML, but I'm not an expert. Please correct me if I'm wrong, smarter people, by replying.

> Please correct me if I'm wrong, smarter people, by replying.

You are not an idiot, maybe not an expert but definitely not an idiot. Your description is quite easy to understand for someone without knowledge in the field. I would add only that RNN are called recurrent because their have recurrent connection with other neurons, and that is why they are hard to parallelize. You need the output the one neuron to compute the output of other neuron in the same layer, so you cannot parallelize that layer. This doesn't happen in CNN.

That's a great explanation.

Let me add this though:

Artificial neural networks were proposed to compute the probability of a sequence of words occurring; however, RNNs were the next step in Natural Language Processing since they allow variable-length sequences to be received as an input contrary to the previously proposed architecture.

However a simple RNN architecture didn't allow for long -term dependencies to be captured (that is, use statistical modeling to predict a word sequence on a part of a text that is based on an idea previously developed on the corpus). So two kinds of fancy RNN architectures were developed to tackle this problem: GRUs and LSTMs. Production systems are already implementing these architectures and they are yielding pretty accurate results.

But now Facebook researchers are proposing using CNNs for this task because this architecture can take more advantage of GPU parallelism.

Not an expert, but as I understand it, common practice (everywhere, not just at Facebook) is to use CNN for understanding images and other kinds of non-sequential data. RNN are commonly used for handling text and other kinds of sequential data.

They showed how to use a CNN with text to get a speed boost, even though that's not how it's normally been done.

As far as I understand, only the use of the attention mechanism with ConvNets is novel, right? Convolutional encoders have been done before.

Yes, there have been a couple of attempts to use CNNs for translation already, but none of them outperformed big and well-tuned LSTM systems. We propose an architecture that is fast to run, easy to optimize and can scale to big networks, and could thus be used as a base architecture for future research.

There are a couple of contributions in the paper (https://arxiv.org/abs/1705.03122) apart from demonstrating the feasibility of CNNs for translation, e.g. the multi-hop attention in combination with a CNN language model, the wiring of the CNN encoder[1], or an initialization scheme for GLUs that, when combined with appropriate scaling for residual connections, enables the training of very deep networks without batch normalization.

[1] In previous work (https://arxiv.org/abs/1611.02344), we required two CNNs in the encoder: one for the keys (dot products) and one for the values (decoder input).

> there have been a couple of attempts to use CNNs for translation already, but none of them outperformed big and well-tuned LSTM systems

It is true that QRNN had results on mostly small-scale benchmarks, but it seemed that Bytenet especially the second version had SOTA results both for language models with characters and for machine translation with characters on the same large-scale En-De WMT task that is used in this paper.

MT with characters, with regards to ordering, structure, etc, is potentially much harder than with words or word-pieces, since the encoded sequences are 5 or 6 times longer on average, and the meanings of words need to be built up from individual characters.

Yes, ByteNet v2 outperforms LSTMs on characters but not on word pieces. It would be interesting to see how our model performs on characters, especially when scaled up to the size of ByteNet (30+30 layers) and also how ByteNet performs on BPE codes. I think that character-level NMT is definitely interesting and worth investigating, but from a practical point of view it makes sense to choose a representation that achieves the maximum translation accuracy and speed.

In this work Convolution Neural Nets (spatial models that have a weakly ordered context, as opposed to Recurrent Neural Nets which are sequential models that have a strongly ordered context) are demonstrated here to achieve State of the Art results in Machine Translation.

It seems the combination of gated linear units / residual connections / attention was the key to bringing this architecture to State of the Art.

It's worth noting that previously the QRNN and ByteNet architectures have used Convolutional Neural Nets for machine translation also. IIRC, those models performed well on small tasks but were not able to best SotA performance on larger benchmark tasks.

I believe it is almost always more desirable to encode a sequence using a CNN if possible as many operations are embarrassingly parallel!

The bleu scores in this work were the following:

Task (previous baseline): new baseline

WMT’16 English-Romanian (28.1): 29.88 WMT’14 English-German (24.61): 25.16 WMT’14 English-French (39.92): 40.46

This smells of "we built custom silicon to do fast image processing using CNNs and fully connected networks, and now we want to use that same silicon for translations. "

I was reading about SyntaxNet (I believe an RNN) developed by Google yesterday. One interesting problem they've run into is getting the system to properly interpret ambiguities. They use the example sentence "Alice drove down the street in her car":

"The first [possible interpretation] corresponds to the (correct) interpretation where Alice is driving in her car; the second [possible interpretation] corresponds to the (absurd, but possible) interpretation where the street is located in her car. The ambiguity arises because the preposition in can either modify drove or street; this example is an instance of what is called prepositional phrase attachment ambiguity."[1]

One thing I believe helps humans interpret these ambiguities is the ability to form visuals from language. A NN that could potentially interpret/manipulate images and decode language seems like it could help solve the above problem and also be applied to a great deal of other things. I imagine (I know embarrassingly little about NNs) this would also introduce a massive amount of complexity.

[1] https://research.googleblog.com/2016/05/announcing-syntaxnet...

I wonder if they can combine this with bytenet (dilated convolutons in place of vanilla convs) - gives you a larger FOV and add in attention and then you probably have a new SOTA.

This is a very cool development. Has anyone written a pytorch or Keras version of the architecture?

Does this mean that we're close to being able to use CNNs for text-to-speech?

no demo?

Yeah I was searching for one as well. I hope someone can link the demo page if possible. Want to see the comparison between systran, Google and FB

There is no online demo but you can run the pre-trained models on your local machine: https://github.com/facebookresearch/fairseq#quick-start. CPU-only versions of the models are available as well.

For a comparison with other translation services, keep in mind that our models have been trained on publicly available news data exclusively, e.g. this corpus for English-French: http://statmt.org/wmt14/translation-task.html#Download .

TLDR: Cutting edge accuracy, nine times faster than previous state of the art, published models and source code.

But go read the article- nice animated diagrams in there.

Haha, this really was TLDR-length. Kudos!

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