Welcome to ShortScience.org! |

- ShortScience.org is a platform for post-publication discussion aiming to improve accessibility and reproducibility of research ideas.
- The website has 1584 public summaries, mostly in machine learning, written by the community and organized by paper, conference, and year.
- Reading summaries of papers is useful to obtain the perspective and insight of another reader, why they liked or disliked it, and their attempt to demystify complicated sections.
- Also, writing summaries is a good exercise to understand the content of a paper because you are forced to challenge your assumptions when explaining it.
- Finally, you can keep up to date with the flood of research by reading the latest summaries on our Twitter and Facebook pages.

Sequence-to-Sequence Learning as Beam-Search Optimization

Sam Wiseman and Alexander M. Rush

arXiv e-Print archive - 2016 via Local arXiv

Keywords: cs.CL, cs.LG, cs.NE, stat.ML

**First published:** 2016/06/09 (7 years ago)

**Abstract:** Sequence-to-Sequence (seq2seq) modeling has rapidly become an important
general-purpose NLP tool that has proven effective for many text-generation and
sequence-labeling tasks. Seq2seq builds on deep neural language modeling and
inherits its remarkable accuracy in estimating local, next-word distributions.
In this work, we introduce a model and beam-search training scheme, based on
the work of Daume III and Marcu (2005), that extends seq2seq to learn global
sequence scores. This structured approach avoids classical biases associated
with local training and unifies the training loss with the test-time usage,
while preserving the proven model architecture of seq2seq and its efficient
training approach. We show that our system outperforms a highly-optimized
attention-based seq2seq system and other baselines on three different sequence
to sequence tasks: word ordering, parsing, and machine translation.
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Sam Wiseman and Alexander M. Rush

arXiv e-Print archive - 2016 via Local arXiv

Keywords: cs.CL, cs.LG, cs.NE, stat.ML

[link]
**Problem Setting:** Sequence to Sequence learning (seq2seq) is one of the most successful techniques in machine learning nowadays. The basic idea is to encode a sequence into a vector (or a sequence of vectors if using attention based encoder) and then use a recurrent decoder to decode the target sequence conditioned on the encoder output. While researchers have explored various architectural changes to this basic encoder-decoder model, the standard way of training such seq2seq models is to maximize the likelihood of each successive target word conditioned on the input sequence and the *gold* history of target words. This is also known as *teacher-forcing* in RNN literature. Such an approach has three major issues: 1. **Exposure Bias:** Since we teacher-force the model with *gold* history during training, the model is never exposed to its errors during training. At test time, we will not have access to *gold* history and we feed the history generated by the model. If it is erroneous, the model does not have any clue about how to rectify it. 2. **Loss-Evaluation Mismatch:** While we evaluate the model using sequence level metrics (such as BLEU for Machine Translation), we are training the model with word level cross entropy loss. 3. **Label bias:** Since the word probabilities are normalized at each time step (by using softmax over the final layer of the decoder), this can result in label bias if we vary the number of possible candidates in each step. More about this later. **Solution:** This paper proposes an alternative training procedure for seq2seq models which attempt to solve all the 3 major issues listed above. The idea is to pose seq2seq learning as beam-search optimization problem. Authors begin by removing the final softmax activation function from the decoder. Now instead of probability distributions, we will get score for next possible word. Then the training procedure is changed as follows: At every time step $t$, they maintain a set $S_t$ of $K$ candidate sequences of length $t$. Now the loss function is defined with the following characteristics: 1. If the *gold* sub-sequence of length $t$ is in set $S_t$ and the score for *gold* sub-sequence exceeds the score of the $K$-th ranked candidate by a margin, the model incurs no loss. Now the candidates for next time-step are chosen in a way similar to regular beam-search with beam-size $K$. 2. If the *gold* sub-sequence of length $t$ is in set $S_t$ and it is the $K$-th ranked candidate, then the loss will push the *gold* sequence up by increasing its score. The candidates for next time-step are chosen in a way similar as first case. 3. If the *gold* sub-sequence of length $t$ is NOT in set $S_t$, then the score of the *gold* sequence is increased to be higher than $K$-th ranked candidate by a margin. In this case, candidates for next step or chosen by only considering *gold* word at time $t$ and getting its top-$K$ successors. 4. Further, since we want the full *gold* sequence to be at top of the beam at the end of the search, when $t=T$, the loss is modified to require the score of *gold* sequence to exceed the score of the *highest* ranked incorrect prediction by a margin. This non-probabilistic training method has several advantages: * The model is trained in a similar way it would be tested, since we use beam-search during training as well as testing. Hence this helps to eliminate exposure bias. * The score based loss can be easily scaled by a mistake-specific cost function. For example, in MT, one could use a cost function which is inversely proportional to BLEU score. So there is no loss-evaluation mismatch. * Each time step can have different set of successor words based on any hard constraints in the problem. Note that the model is non-probabilistic and hence this varying successor function will not introduce any label bias. Refer [this set of slides][1] for an excellent illustration of label bias. Cost of forward-prop grows linearly with respect to beam size $K$. However, GPU implementation should help to reduce this cost. Authors propose a clever way of doing BPTT which makes the back-prop almost same cost as ordinary seq2seq training. **Additional Tricks** 1. Authors pre-train the seq2seq model with regular word level cross-entropy loss and this is crucial since random initialization did not work. 2. Authors use "curriculum beam" strategy in training where they start with beam size of 2 and increase the beam size by 1 for every 2 epochs until it reaches the required beam size. You have to train your model with training beam size of at least test beam size + 1. (i.e $K_{tr} >= K_{te} + 1$). 3. When you use drop-out, you need to be careful to use the same dropout value during back-prop. Authors do this by sharing a single dropout across all sequences in a time step. **Experiments** Authors compare the proposed model against basic seq2seq in word ordering, dependency parsing and MT tasks. The proposed model achieves significant improvement over the strong baseline. **Related Work:** The whole idea of the paper is based on [learning as search optimization (LaSO) framework][2] of Daume III and Marcu (2005). Other notable related work are training seq2seq models using mix of cross-entropy and REINFORCE called [MIXER][3] and [an actor-critic based seq2seq training][4]. Authors compare with MIXER and they do significantly better than MIXER. **My two cents:** This is one of the important research directions in my opinion. While other recent methods attempt to use reinforcement learning to avoid the issues in word-level cross-entropy training, this paper proposes a really simple score based solution which works very well. While most of the language generation research is stuck with probabilistic framework (I am saying this w.r.t Deep NLP research), this paper highlights the benefits on non-probabilistic generation models. I see this as one potential way of avoiding the nasty scalability issues that come with softmax based generative models. [1]: http://www.cs.stanford.edu/~nmramesh/crf [2]: https://www.isi.edu/~marcu/papers/daume05laso.pdf [3]: http://arxiv.org/pdf/1511.06732v7.pdf [4]: https://arxiv.org/pdf/1607.07086v2.pdf |

Towards a Neural Statistician

Harrison Edwards and Amos Storkey

arXiv e-Print archive - 2016 via Local arXiv

Keywords: stat.ML, cs.LG

**First published:** 2016/06/07 (7 years ago)

**Abstract:** An efficient learner is one who reuses what they already know to tackle a new
problem. For a machine learner, this means understanding the similarities
amongst datasets. In order to do this, one must take seriously the idea of
working with datasets, rather than datapoints, as the key objects to model.
Towards this goal, we demonstrate an extension of a variational autoencoder
that can learn a method for computing representations, or statistics, of
datasets in an unsupervised fashion. The network is trained to produce
statistics that encapsulate a generative model for each dataset. Hence the
network enables efficient learning from new datasets for both unsupervised and
supervised tasks. We show that we are able to learn statistics that can be used
for: clustering datasets, transferring generative models to new datasets,
selecting representative samples of datasets and classifying previously unseen
classes.
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Harrison Edwards and Amos Storkey

arXiv e-Print archive - 2016 via Local arXiv

Keywords: stat.ML, cs.LG

[link]
This paper can be thought as proposing a variational autoencoder applied to a form of meta-learning, i.e. where the input is not a single input but a dataset of inputs. For this, in addition to having to learn an approximate inference network over the latent variable $z_i$ for each input $x_i$ in an input dataset $D$, approximate inference is also learned over a latent variable $c$ that is global to the dataset $D$. By using Gaussian distributions for $z_i$ and $c$, the reparametrization trick can be used to train the variational autoencoder. The generative model factorizes as $p(D=(x_1,\dots,x_N), (z_1,\dots,z_N), c) = p(c) \prod_i p(z_i|c) p(x_i|z_i,c)$ and learning is based on the following variational posterior decomposition: $q((z_1,\dots,z_N), c|D=(x_1,\dots,x_N)) = q(c|D) \prod_i q(z_i|x_i,c)$. Moreover, latent variable $z_i$ is decomposed into multiple ($L$) layers $z_i = (z_{i,1}, \dots, z_{i,L})$. Each layer in the generative model is directly connected to the input. The layers are generated from $z_{i,L}$ to $z_{i,1}$, each layer being conditioned on the previous (see Figure 1 *Right* for the graphical model), with the approximate posterior following a similar decomposition. The architecture for the approximate inference network $q(c|D)$ first maps all inputs $x_i\in D$ into a vector representation, then performs mean pooling of these representations to obtain a single vector, followed by a few more layers to produce the parameters of the Gaussian distribution over $c$. Training is performed by stochastic gradient descent, over minibatches of datasets (i.e. multiple sets $D$). The model has multiple applications, explored in the experiments. One is of summarizing a dataset $D$ into a smaller subset $S\in D$. This is done by initializing $S\leftarrow D$ and greedily removing elements of $S$, each time minimizing the KL divergence between $q(c|D)$ and $q(c|S)$ (see the experiments on a synthetic Spatial MNIST problem of section 5.3). Another application is few-shot classification, where very few examples of a number of classes are given, and a new test example $x'$ must be assigned to one of these classes. Classification is performed by treating the small set of examples of each class $k$ as its own dataset $D_k$. Then, test example $x$ is classified into class $k$ for which the KL divergence between $q(c|x')$ and $q(c|D_k)$ is smallest. Positive results are reported when training on OMNIGLOT classes and testing on either the MNIST classes or unseen OMNIGLOT datasets, when compared to a 1-nearest neighbor classifier based on the raw input or on a representation learned by a regular autoencoder. Finally, another application is that of generating new samples from an input dataset of examples. The approximate posterior is used to compute $q(c|D)$. Then, $c$ is assigned to its posterior mean, from which a value for the hidden layers $z$ and finally a sample $x$ can be generated. It is shown that this procedure produces convincing samples that are visually similar from those in the input set $D$. **My two cents** Another really nice example of deep learning applied to a form of meta-learning, i.e. learning a model that is trained to take *new* datasets as input and generalize even if confronted to datasets coming from an unseen data distribution. I'm particularly impressed by the many tasks explored successfully with the same approach: few-shot classification and generative sampling, as well as a form of summarization (though this last probably isn't really meta-learning). Overall, the approach is quite elegant and appealing. The very simple, synthetic experiments of section 5.1 and 5.2 are also interesting. Section 5.2 presents the notion of a *prior-interpolation layer*, which is well motivated but seems to be used only in that section. I wonder how important it is, outside of the specific case of section 5.2. Overall, very excited by this work, which further explores the theme of meta-learning in an interesting way. |

Sanity Checks for Saliency Maps

Julius Adebayo and Justin Gilmer and Michael Muelly and Ian Goodfellow and Moritz Hardt and Been Kim

arXiv e-Print archive - 2018 via Local arXiv

Keywords: cs.CV, cs.LG, stat.ML

**First published:** 2018/10/08 (5 years ago)

**Abstract:** Saliency methods have emerged as a popular tool to highlight features in an
input deemed relevant for the prediction of a learned model. Several saliency
methods have been proposed, often guided by visual appeal on image data. In
this work, we propose an actionable methodology to evaluate what kinds of
explanations a given method can and cannot provide. We find that reliance,
solely, on visual assessment can be misleading. Through extensive experiments
we show that some existing saliency methods are independent both of the model
and of the data generating process. Consequently, methods that fail the
proposed tests are inadequate for tasks that are sensitive to either data or
model, such as, finding outliers in the data, explaining the relationship
between inputs and outputs that the model learned, and debugging the model. We
interpret our findings through an analogy with edge detection in images, a
technique that requires neither training data nor model. Theory in the case of
a linear model and a single-layer convolutional neural network supports our
experimental findings.
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Julius Adebayo and Justin Gilmer and Michael Muelly and Ian Goodfellow and Moritz Hardt and Been Kim

arXiv e-Print archive - 2018 via Local arXiv

Keywords: cs.CV, cs.LG, stat.ML

[link]
**Idea:** With the growing use of visual explanation systems of machine learning models such as saliency maps, there needs to be a standardized method of verifying if a saliency method is correctly describing the underlying ML model. **Solution:** In this paper two Sanity Checks have been proposed to verify the accuracy and the faithfulness of a saliency method: * *Model parameter randomization test:* In this sanity check the outputs of a saliency method on a trained model is compared to that of the same method on an untrained randomly parameterized model. If these images are similar/identical then this saliency method does not correctly describe the model. In the course of this experiment it is found that certain methods such as the Guided BackProp are constant in their explanations despite alterations in the model. * *Data Randomization Test:* This method explores the relationship of saliency methods to data and their associated labels. In this test, the labels of the training data are randomized thus there should be no definite pattern describing the model (Since the model is as good as randomly guessing an output label). If there is a definite pattern, this shows that the saliency methods are independent of the underlying model/training data labels. In this test as well Guided BackProp did not fare well, implying this saliency method is as good as an edge detector as opposed to a ML explainer. Thus this paper makes a valid argument toward having standardized tests that an interpretation model must satisfy to be deemed accurate or faithful. |

A Meta-Transfer Objective for Learning to Disentangle Causal Mechanisms

Yoshua Bengio and Tristan Deleu and Nasim Rahaman and Rosemary Ke and Sébastien Lachapelle and Olexa Bilaniuk and Anirudh Goyal and Christopher Pal

arXiv e-Print archive - 2019 via Local arXiv

Keywords: cs.LG, stat.ML

**First published:** 2019/01/30 (5 years ago)

**Abstract:** We propose to meta-learn causal structures based on how fast a learner adapts
to new distributions arising from sparse distributional changes, e.g. due to
interventions, actions of agents and other sources of non-stationarities. We
show that under this assumption, the correct causal structural choices lead to
faster adaptation to modified distributions because the changes are
concentrated in one or just a few mechanisms when the learned knowledge is
modularized appropriately. This leads to sparse expected gradients and a lower
effective number of degrees of freedom needing to be relearned while adapting
to the change. It motivates using the speed of adaptation to a modified
distribution as a meta-learning objective. We demonstrate how this can be used
to determine the cause-effect relationship between two observed variables. The
distributional changes do not need to correspond to standard interventions
(clamping a variable), and the learner has no direct knowledge of these
interventions. We show that causal structures can be parameterized via
continuous variables and learned end-to-end. We then explore how these ideas
could be used to also learn an encoder that would map low-level observed
variables to unobserved causal variables leading to faster adaptation
out-of-distribution, learning a representation space where one can satisfy the
assumptions of independent mechanisms and of small and sparse changes in these
mechanisms due to actions and non-stationarities.
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Yoshua Bengio and Tristan Deleu and Nasim Rahaman and Rosemary Ke and Sébastien Lachapelle and Olexa Bilaniuk and Anirudh Goyal and Christopher Pal

arXiv e-Print archive - 2019 via Local arXiv

Keywords: cs.LG, stat.ML

[link]
How can we learn causal relationships that explain data? We can learn from non-stationary distributions. If we experiment with different factorizations of relationships between variables we can observe which ones provide better sample complexity when adapting to distributional shift and therefore are likely to be causal. If we consider the variables A and B we can factor them in two ways: $P(A,B) = P(A)P(B|A)$ representing a causal graph like $A\rightarrow B$ $P(A,B) = P(A|B)P(B)$ representing a causal graph like $A \leftarrow B$ The idea is if we train a model with one of these structures; when adapting to a new shifted distribution of data it will take longer to adapt if the model does not have the correct inductive bias. For example let's say that the true relationship is $A$=Raining causes $B$=Open Umbrella (and not vice-versa). Changing the marginal probability of Raining (say because the weather changed) does not change the mechanism that relates $A$ and $B$ (captured by $P(B|A)$), but will have an impact on the marginal $P(B)$. So after this distributional shift the function that modeled $P(B|A)$ will not need to change because the relationship is the same. Only the function that modeled $P(A)$ will need to change. Under the incorrect factorization $P(B)P(A|B)$, adaptation to the change will be slow because both $P(B)$ and $P(A|B)$ need to be modified to account for the change in $P(A)$ (due to Bayes rule). Here a difference in sample complexity can be observed when modeling the joint of the shifted distribution. $B\rightarrow A$ takes longer to adapt: https://i.imgur.com/B9FEmA7.png Here the idea is that sample complexity when adapting to a new distribution of data is a heuristic to inform us which causal graph inductive bias is correct. Experimentally this works and they also observe that when models have more capacity it seems that the difference between the models grows. This summary was written with the help of Yoshua Bengio. |

Analyzing Multi-Head Self-Attention: Specialized Heads Do the Heavy Lifting, the Rest Can Be Pruned

Elena Voita and David Talbot and Fedor Moiseev and Rico Sennrich and Ivan Titov

arXiv e-Print archive - 2019 via Local arXiv

Keywords: cs.CL

**First published:** 2019/05/23 (4 years ago)

**Abstract:** Multi-head self-attention is a key component of the Transformer, a
state-of-the-art architecture for neural machine translation. In this work we
evaluate the contribution made by individual attention heads in the encoder to
the overall performance of the model and analyze the roles played by them. We
find that the most important and confident heads play consistent and often
linguistically-interpretable roles. When pruning heads using a method based on
stochastic gates and a differentiable relaxation of the L0 penalty, we observe
that specialized heads are last to be pruned. Our novel pruning method removes
the vast majority of heads without seriously affecting performance. For
example, on the English-Russian WMT dataset, pruning 38 out of 48 encoder heads
results in a drop of only 0.15 BLEU.
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Elena Voita and David Talbot and Fedor Moiseev and Rico Sennrich and Ivan Titov

arXiv e-Print archive - 2019 via Local arXiv

Keywords: cs.CL

[link]
In the two years since it’s been introduced, the Transformer architecture, which uses multi-headed self-attention in lieu of recurrent models to consolidate information about input and output sequences, has more or less taken the world of language processing and understanding by storm. It has become the default choice for language for problems like translation and questioning answering, and was the foundation of OpenAI’s massive language-model-trained GPT. In this context, I really appreciate this paper’s work to try to build our collective intuitions about the structure, specifically by trying to understand how the multiple heads that make up the aforementioned multi-head attention divide up importance and specialize function. As a quick orientation, attention works by projecting each value in the sequence into query, key, and value vectors. Then, each element in the sequence creates its next-layer value by calculating a function of query and key (typically dot product) and putting that in a softmax against the query results with every other element. This weighting distribution is then used as the weights of a weighted average, combining the values together. By default this is a single operation, with a single set of projection matrices, but in the Transformer approach, they use multi-headed attention, which simply means that they learn independent parameters for multiple independent attention “filters”, each of which can then notionally specialize to pull in and prioritize a certain kind of information. https://i.imgur.com/yuC91Ja.png The high level theses of this paper are: - Among attention heads, there’s a core of the most crucial and important ones, and then a long tail of heads that can be pruned (or, have their weight in the concatenation of all attention heads brought to nearly zero) and have a small effect on performance - It’s possible and useful to divide up the heads according to the kinds of other tokens that it is most consistently pulling information from. The authors identify three: positional, syntactic, and rare. Positional heads consistently (>90% of the time) put their maximum attention weight on the same position relative to the query word. Syntactic heads are those that recurringly in the same grammatical relation to the query, the subject to its verb, or the adjective to its noun, for example. Rare words is not a frequently used head pattern, but it is a very valuable head within the first layer, and will consistently put its highest weight on the lowest-frequency word in a given sentence. An interesting side note here is that the authors tried at multiple stages in pruning to retrain a network using only the connections between unpruned heads, and restarting from scratch. However, in a effect reminiscent of the Lottery Ticket Thesis, retraining from scratch cannot get quite the same performance. |

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