Robust NMT

Neural machine translation (NMT) often suffers from the vulnerability to noisy perturbation in the input. Google AI has proposed an approach to improving the robustness of NMT models called AdvGen published in 2019 in their paper: Robust Neural Machine Translation with Doubly Adversarial Inputs. AdvGen consists of two parts:

Attack the translation model with adversarial source examples.
Defend the translation model with adversarial target inputs to improve its robustness against the adversarial source inputs.

This approach achieves significant improvements over the previous state-of-the-art Transformer model on two common translation benchmarks: Chinese-English and English-German which substantiates that the proposed model improves the generalization performance over the clean benchmark datasets.

AdvGen

AdvGen is a gradient-based approach to construct adversarial examples and use these examples to both attack as well as defend the NMT model. The intuition behind it is that an ideal model would generate similar translation results for similar input sentences despite any small difference caused by perturbations.

Attack Source Input

Given a parallel sentence pair $(x,\ y)$, For the original input $x$, we induce a possible adversarial word ${x’}{i}$ for the token $x{i}$ in x:

\[{x'}_{i} = \underset{x \in \mathcal{V}_{x_{i}}}{\arg\max}\left( \text{sim}\left( e\left( x \right) - e\left( x_{i} \right) \right),\ g_{x_{i}} \right)\]

Where:

$e\left( x_{i} \right)$ is the embedding of token $x_{i}$.
$g_{x_{i}}$ is a gradient vector with respect to $e\left( x_{i} \right)$:

\[g_{x_{i}} = \frac{\partial}{\partial e\left( x_{i} \right)} - logP\left( y \middle| x;\ \theta \right)\]

$sim( \cdot ,\ \cdot )$ denotes the similarity function by calculating the cosine distance between two vectors.
$\mathcal{V}_{x_{i}}$ is a subset of the vocabulary for the source language that is specific for each token $x_{i}$ is defined as the n most probable words among the top n scores calculated by $a$ is a bidirectional language model for the source language $P_{\text{lm}}$:

\[\mathcal{V}_{x_{i}} = top\_ n\left( Q\left( x_{i},\ x \right) \right);\ \ \ \ Q\left( x_{i},\ x \right) = P_{\text{lm}}\left( x \middle| x_{< i},\ x_{> i};\ \theta_{\text{lm}}^{x} \right)\]

More formally, the AdvGen function can be written as an algorithm like so:

Defense Target Input

After generating an adversarial example $x’$, we treat $(x’,\ y)$ as a new training data point to improve the model’s robustness. These adversarial examples in the source tend to introduce errors which may accumulate and cause drastic changes to the decoder prediction. To defend the model from errors in the decoder predictions, AdvGen generates an adversarial target input.

Formally, let $z$ be the decoder input for the sentence pair $(x,\ y)$. Using the same AdvGen function, we will generate an adversarial target input $z’$ from $z$ by:

\[z' = \text{AdvGen}\left( z,\ Q_{\text{trg}},\ D_{\text{trg}},\ - \log P\left( y \middle| x' \right) \right)\]

Note that:

For the target, the translation loss in the attack part $- logP\left( y \middle| x;\ \theta \right)$ is replaced by $- logP\left( y \middle| x’ \right)$.
$Q_{\text{trg}}$ is the likelihood for selecting the target word candidate set $\mathcal{V}_{z}$. To compute it, we combine the NMT model prediction $P_{\text{nmt}}$ with a bidirectional language model $P_{\text{lm}}$ for the target language with a tunable hyper-parameter $\lambda$ that balances the importance between two models as follow:

\[Q_{\text{trg}}\left( z_{i},\ z \right) = \lambda P_{\text{lm}}\left( z \middle| z_{< i},\ z_{> i};\ \theta_{\text{lm}}^{y} \right) + \left( 1 - \lambda \right)P_{\text{nmt}}\left( z \middle| z_{< i},\ x';\ \theta_{\text{nmt}} \right)\]

$D_{\text{trg}}$ is a distribution for sampling positions for the target input. Different from the uniform distribution used in the source, in the target sentence we want to change those relevant words influenced by the perturbed words in the source input. To do so, we use the attention matrix $M$ learned in the NMT model, obtained at the current mini-batch, to compute the distribution over $(x,\ y,\ x’)$ by:

\[P\left( j \right) = \frac{\sum_{i}^{}M_{\text{ij}}\ \delta\left( x_{i},\ {x'}_{i} \right)}{\sum_{k}^{}{\sum_{i}^{}M_{\text{ik}}\ \delta\left( x_{i},\ {x'}_{i} \right)}},\ \ \ j \in \left\lbrack 1,\ ...,\ \left| y \right| \right\rbrack\]

Where $M_{\text{ij}}$ is the attention score between $x_{i}$ and $y_{j}$; and $\delta\left( x_{i},\ {x’}_{i} \right)$ is an indicator function that yields 1 if $x_{i} \neq {x’}_{i}$ and 0 otherwise.

Robustness Loss

This algorithm details the entire procedure to calculate the robustness loss for a parallel sentence pair $(x,\ y)$. This algorithm takes at most a 20% time overhead compared to the standard Transformer model.

Accordingly, we compute the robustness loss on $S$ as:

\[\mathcal{L}_{\text{robust}}\left( \theta_{\text{nmt}} \right) = \frac{1}{\left| S \right|}\sum_{\left( x,y \right) \in S}^{}{- \log P\left( y \middle| x',\ z';\ \theta_{\text{nmt}} \right)}\]

And the final loss will be a combination of four loss functions:

\[\mathcal{L}\left( \theta_{\text{nmt}},\ \theta_{\text{lm}}^{x},\ \theta_{\text{lm}}^{y} \right) = \mathcal{L}_{\text{clean}}\left( \theta_{\text{nmt}} \right) + \mathcal{L}_{\text{lm}}\left( \theta_{\text{lm}}^{x} \right) + \mathcal{L}_{\text{robust}}\left( \theta_{\text{nmt}} \right) + \mathcal{L}_{\text{lm}}\left( \theta_{\text{lm}}^{y} \right)\]

Where:

$\mathcal{L}_{\text{lm}}\left( \theta_{\text{lm}}^{x} \right)$ and $\mathcal{L}_{\text{lm}}\left( \theta_{\text{lm}}^{y} \right)$ are loss functions for source and target bidirectional language models, respectively.
$\mathcal{L}_{\text{clean}}\left( \theta_{\text{nmt}} \right)$ is the loss function for the NMT model.
$\mathcal{L}_{\text{robust}}\left( \theta_{\text{nmt}} \right)$ is the robustness loss.