# ICLR 2020

## Variational Template Machine for Data-to-Text Generation

- A graphical model for generating text $y$ from structured data $x$
- Similar to a variational autoencoder, but adds latent variables that represent a template $z$ and content $c$
- Continuous latent variables generate diverse output
- Reconstruction loss for output given data and template
- Template preserving loss: template variable can reconstruct the text

### Variational Inference

- Approximation for Bayesian inference with graphical models
- Computing posterior probabilities is hard because of the integration required to compute the evidence probability
- Approximates the posterior distribution using a variational posterior from a family of distributions
- Computing the KL divergence between the approximate and exact posterior would be equally hard, since it depends on the evidence probability
- Minimizing KL divergence is equal to minimizing the negative evidence lower bound (ELBO)
- ELBO is log evidence subtracted from the KL divergence
- The optimization problem is not as hard as integration

## Meta-Learning with Warped Gradient Descent

- Model consists of task layers and warp layers
- Task layers are updated using a normal loss
- Meta-learn warp-layers that modify the loss surface to be smoother
- Novelty: non-linear warp layers avoid dependence on the trajectory or the initialization, depending only on the task parameter updates at the current position of the search space

### Meta-Learning

- Learning some parameters of the optimizer such as initialization
- Optimize for the final model accuracy as a function of the initialization
- When using gradient descent, the objective is differentiable
- Backpropagate through all training steps back into the initialization
- Vanishing / exploding gradients when the loss surface is very flat or steep
- Costly—usually scales to a handful of training steps only

## Transformer-XH

- Adaptation of Transformer for structured text
- For example, multi-evidence reasoning: answer questions, following words that are linked to Wikipedia
- Extra hop attention attends to the first token of another sequence (representative of the entire sequence)

## Deep Double Descent

- Bigger model means lower training loss
- At some point test error starts to increase, but with large enough models decreases again
- Occurs across various architectures (CNNs, ResNets, Transformers), data domains (NLP, vision), and optimizer (SGD, Adam)
- Also occurs when increasing training time
- In some cases increasing training data can hurt performance
- Effective model complexity takes training time into account

## Towards Stabilizing Batch Statistics in Backward Propagation of Batch Normalization

- Unstable mean and variance estimation with too small batch sizes
- Batch renormalization (BRN): corrects batch mean and variance by moving average
- Moving average batch normalization: moving average of variance mean reduction + weight centralization

## Compressive Transformer for Long-Range Sequence Modelling

- Train a language model on segments similar to Transformer-XL
- When moving to the next segment, the oldest N activations in the memory are compressed using a compressing function

### Related Work

- Lightweight and Dynamic Convolution: depth-wise separable convolution that runs in linear time
- Transformer-XL: train a language model on segments, but include activations from the previous segment in “extended context”
- Sparse Transformer: sparse attention masks
- Adaptive Attention Span: different attention heads can have longer or shorter spans of attention

## A Closer Look at Deep Policy Gradients

- Policy gradient methods: optimize policy parameters to maximize the expected reward
- Variance reduction using baseline—separate the quality of the action from the quality of the state
- A canonical choice of baseline function is the value function
- Surrogate objective is a simplification of reward maximization used by modern policy gradient algorithms (policy is divided by the old policy, Schulman et al. 2015)
- Measure of gradient variance: mean pairwise correlation (similarity) between gradient samples
- Visualization of optimization landscapes with different number of samples per estimate

## Laurent Dinh: Invertible Models and Normalizing Flows

- A model finds a representation $h = f(x)$ of a datapoint $x$ (in practice, an image)
- Generative models (VAE, GAN) can produce an image $x$ from its representation $h$
*Normalizing flow*is a sequence of invertible transformations- Reversible generative models can encode an image into a latent space, making it possible to interpolate between two images

### Variational Autoencoder (VAE)

- A standard autoencoder learns representations with distinct clusters in the latent space
- VAE encoder produces a set of means and variances, and then samples the inputs of the decoder
- The latent space is continuous
- Only approximate inference of latent variables from a datapoint

### Generative Adversarial Network (GAN)

- A generator network creates fake images and a discriminator network learns to distinguish them from real images
- Images cannot be encoded into the latent space

### NICE

- Finds a representation such that $p(h)$ factorizes as $\prod p(h_d)$, where $h_d$ are independent latent variables (arguably a “good” representation)
- Prior distribution $p(h_d)$ is Gaussian or logistic
- Training: maximize the likelihood of the data using a “change of variables” formula
- $f(x)$ has to be invertible—achieved by splitting $x$ into $x_1$ and $x_2$ and using a transformation that transforms them into $x_1$ and $x_2 + m(x_1)$ respectively
- Sampling images: sample from $p(h)$ and use the inverse of $f(x)$

## A Probabilistic Formulation of Unsupervised Text Style Transfer

- Change text style, keeping the semantic meaning unchanged
- Machine translation, sentiment transfer (positive ↔ negative)
- Unsupervised (only a nonparallel corpus)
- Previous work on text style transfer: autoencoding loss + adversarial loss using a language model as a discriminator
- Previous work on machine translation: cycle structures for unsupervised machine translation
- Novelty: probabilistic formulation of the above heuristic training strategies
- Translations of language A are “latent sequences” of language B and vice versa
- We want to utilize a language model prior within each language
- Train using variational inference

## Estimating Gradients for Discrete Random Variables by Sampling without Replacement

- Strategies for obtaining gradients for discrete outputs: smoothening the outputs (relaxation), or sampling (REINFORCE)
- REINFORCE: move the gradient inside the expectation and estimate it using a sample
- With multiple samples one can use the average as a baseline
- Sampling without replacement can be done efficiently by taking the top $k$ of Gumbel variables
- Probability for sampling an unordered sample can be calculated as a sum over all possible permutations
- The estimator is changed to work with an unordered set

## Reformer

- Memory efficiency: reversible residual layers, chunked FF and attention layers
- Time complexity: attention within buckets created using locality sensitive hashing

## A Theoretical Analysis of the Number of Shots in Few-Shot Learning

- Prototypical networks cannot handle different numbers of shots between classes
- Performance drops when there’s a mismatch in the number of shots between meta-training and testing
- Trade-off between minimizing intra-class variance and maximizing inter-class variance is different when clustering a different number of embeddings
- Proposes an embedding space transformation

### Prototypical Networks

- Aggregate experiences from learning other tasks to learn a few-shot task
- Form prototypes of each class as the average embedding of the labeled “support” examples
- Class likelihoods from the distances from the embedding of the current example to the prototypes

## Mixed Precision DNNs

- Fixed-point representation for the weights and activations, with a different bitwidth for each layer
- A quantizer DNN is learned using gradient-based methods
- Which parameters (bitwidth, step size, minimum value, maximum value) to use for parameterization of uniform and power-of-two quantizations?
- The gradients with regard to the quantizer parameters are bounded and decoupled when choosing step size and maximum value, or minimum value and maximum value
- How to learn the parameters?
- A penalty term is added to the loss to enforce size constraints for the weights and activations

## Training Binary Neural Networks with Real-to-Binary Convolutions

- Binary convolution can be implemented using fast xnor and pop-count operations
- Per-channel scaling is used to produce real-valued outputs
- Teacher-student with a real-valued teacher

## On Mutual Information Maximization for Representation Learning

- Unsupervised learning based on information theoretic concepts
- InfoMax principle: a good representation should have high mutual information with the input
- MMI alone is not sufficient for representation learning, but modern methods work well in practice
- Multi-view approach: maximize mutual information between different views of the same input
- For example: split an image in half, encode both parts independently, and compare the mutual information between the parts
- If the representation encodes high-level features of the image, mutual information will be high; if it encodes noise, mutual information will be low

## A Mutual Information Maximization Perspective of Language Representation Learning

- Many language tasks can be formulated as maximizing an objective function that is a lower bound on mutual information between different parts of the text sequence
- BERT (masked LM): word and corrupted word context, or sentence and following sentence
- Skip-gram (word2vec): word and word context
- InfoWorld (proposed by the authors): sentence and n-gram, both encoded using Transformer

### Mutual Information Neural Estimation

- Mutual information: the amount the uncertainty about $X$ is reduced by knowing the value of $Z$
- Maximizing mutual information directly is infeasible
- The mutual information between $X$ and $Z$ can be expressed as the KL divergence between the joint probability distribution and the product of the marginal distributions: $I(X;Z) = D_{KL}(P_{XZ} \parallel P_X P_Z)$
- $E_P[f(x)] - \log E_Q[e^{f(x)}]$, where $f(x)$ is any real-valued function for which the expectations are finite, is always less than or equal to the KL divergence between $P$ and $Q$
- Donsker-Varadhan representation for KL divergence: supremum of this lower bound is equal to the KL divergence
- In theory, any function can be represented with a neural network ⇒ we can train a neural network $f(x)$ to maximize the lower bound

### InfoNCE

- Maximize mutual information between target $x$ and context $c$
- One positive sample from $p(x \mid c)$ and $N-1$ negative samples from $p(x)$
- Loss based on noise contrastive estimation

## ALBERT

- Small “sandwich” layer reduce the number of embedding parameters
- By default shares all parameters between layers
- Additional next-sentence prediction loss
- Dropout removed
- More data

## Incorporating BERT into Neural Machine Translation

- Initialize weights of NMT encoder from BERT (degradation)
- Initialize encoder and decoder weights with cross-lingual BERT trained on a multilingual corpus (small improvement)
- Create embeddings using BERT (significant improvement)
- BERT-fused NMT: additional attention to BERT (whose parameters are fixed) in each layer
- Drop-net trick: with certain probability perform a regularization step—use only BERT-encoder attention or self-attention
- SOTA results in semi-supervised NMT

## Mixout

- Weight decay towards previous model parameters prevents catastrophic forgetting on the pretrained task
- Mixout sets parameters from a randomly selected neuron to those of the pretrained model during fine-tuning
- Corresponds to adaptive weight decay towards the pretrained model

## Network Deconvolution

- There is a lot of correlation between nearby pixels, even when an image is not blurred
- Correlation in data causes gradient descent to take more steps
- Correlation between dimensions can be removed with a coordinate transform
- Calculate the correlation at every layer and apply inverse filtering
- Results in a sparse representation

## A Signal Propagation Perspective for Pruning Neural Networks at Initialization

- By repeatedly training and pruning connections, model size can be reduced
- Even randomly initialized networks can be pruned prior to training, based on connection sensitivity
- It is unclear why pruning the initialization is effective
- Scaling of the initialization can have a critical impact

## Monotonic Multihead Attention

- Simultaneous translation: start translating before reading the full input
- Monotonic attention: stepwise probability for decision whether to read a source token or write a target token
- State of the art: Monotonic Infinite Loopback Attention (based on LSTM)
- Novelty: Transformer with multihead monotonic attention
- Independent stepwise probabilities for different heads
- A source token is read if the fastest head decides to read
- A target token is written if all the heads finish reading
- Implemented in Fairseq

## Revisiting Self-Training for Neural Sequence Generation

- Self-training: train a teacher model using labeled data and a student model using the predictions of the teacher on unlabeled data
- Fine-tune the student model on the labeled data
- Helps on machine translation (100k parallel, 3.8M monolingual samples)
- Beam search, when decoding the unlabeled data, contributes a bit to the gain (compared to sampling from the teacher’s output distribution)
- Dropout, while training on the pseudo-data, accounts for most of the gain