Homework and bake-off: word-level entailment with neural networks

Contents

1. Overview
2. Set-up
3. Data
   1. Edge disjoint
   2. Word disjoint
4. Baseline
   1. Representing words: vector_func
   2. Combining words into inputs: vector_combo_func
   3. Classifier model
   4. Baseline results
5. Homework questions
   1. Hypothesis-only baseline [2 points]
   2. Alternatives to concatenation [2 points]
   3. A deeper network [2 points]
   4. Your original system [3 points]
6. Bake-off [1 point]

Overview

The general problem is word-level natural language inference.

Training examples are pairs of words $(w_{L}, w_{R}), y$ with $y = 1$ if $w_{L}$ entails $w_{R}$, otherwise $0$.

The homework questions below ask you to define baseline models for this and develop your own system for entry in the bake-off, which will take place on a held-out test-set distributed at the start of the bake-off. (Thus, all the data you have available for development is available for training your final system before the bake-off begins.)

Set-up

See the first notebook in this unit for set-up instructions.

Data

I've processed the data into two different train/test splits, in an effort to put some pressure on our models to actually learn these semantic relations, as opposed to exploiting regularities in the sample.

• edge_disjoint: The train and dev edge sets are disjoint, but many words appear in both train and dev.
• word_disjoint: The train and dev vocabularies are disjoint, and thus the edges are disjoint as well.

These are very different problems. For word_disjoint, there is real pressure on the model to learn abstract relationships, as opposed to memorizing properties of individual words.

The outer keys are the splits plus a list giving the vocabulary for the entire dataset:

Edge disjoint

This is what the split looks like; all three have this same format:

Let's test to make sure no edges are shared between train and dev:

As we expect, a lot of vocabulary items are shared between train and dev:

This is a large percentage of the entire vocab:

Here's the distribution of labels in the train set. It's highly imbalanced, which will pose a challenge for learning. (I'll go ahead and reveal that the dev set is similarly distributed.)

Word disjoint

In the word_disjoint split, no words are shared between train and dev:

Because no words are shared between train and dev, no edges are either:

The label distribution is similar to that of edge_disjoint, though the overall number of examples is a bit smaller:

Baseline

Even in deep learning, feature representation is vital and requires care! For our task, feature representation has two parts: representing the individual words and combining those representations into a single network input.

Representing words: vector_func

Let's consider two baseline word representations methods:

1. Random vectors (as returned by utils.randvec).
2. 50-dimensional GloVe representations.

Combining words into inputs: vector_combo_func

Here we decide how to combine the two word vectors into a single representation. In more detail, where u is a vector representation of the left word and v is a vector representation of the right word, we need a function vector_combo_func such that vector_combo_func(u, v) returns a new input vector z of dimension m. A simple example is concatenation:

vector_combo_func could instead be vector average, vector difference, etc. (even combinations of those) – there's lots of space for experimentation here; homework question 2 below pushes you to do some exploration.

Classifier model

For a baseline model, I chose TorchShallowNeuralClassifier:

Baseline results

The following puts the above pieces together, using vector_func=glove_vec, since vector_func=randvec seems so hopelessly misguided for word_disjoint!

Homework questions

Please embed your homework responses in this notebook, and do not delete any cells from the notebook. (You are free to add as many cells as you like as part of your responses.)

Hypothesis-only baseline [2 points]

During our discussion of SNLI and MultiNLI, we noted that a number of research teams have shown that hypothesis-only baselines for NLI tasks can be remarkably robust. This question asks you to explore briefly how this baseline effects the 'edge_disjoint' and 'word_disjoint' versions of our task.

For this problem, submit two functions:

1. A vector_combo_func function called hypothesis_only that simply throws away the premise, using the unmodified hypothesis (second) vector as its representation of the example.

2. A function called run_hypothesis_only_evaluation that does the following:

   1. Loops over the two conditions 'word_disjoint' and 'edge_disjoint' and the two vector_combo_func values vec_concatenate and hypothesis_only, calling nli.wordentail_experiment to train on the conditions 'train' portion and assess on its 'dev' portion, with glove_vec as the vector_func. So that the results are consistent, use an sklearn.linear_model.LogisticRegression with default parameters as the model.
   2. Returns a dict mapping (condition_name, function_name) pairs to the 'macro-F1' score for that pair, as returned by the call to nli.wordentail_experiment. (Tip: you can get the str name of your function hypothesis_only with hypothesis_only.__name__.)

The test functions test_hypothesis_only and test_run_hypothesis_only_evaluation will help ensure that your functions have the desired logic.

Alternatives to concatenation [2 points]

We've so far just used vector concatenation to represent the premise and hypothesis words. This question asks you to explore two simple alternative:

1. Write a function vec_diff that, for a given pair of vector inputs u and v, returns the element-wise difference between u and v.

2. Write a function vec_max that, for a given pair of vector inputs u and v, returns the element-wise max values between u and v.

You needn't include your uses of nli.wordentail_experiment with these functions, but we assume you'll be curious to see how they do!

A deeper network [2 points]

It is very easy to subclass TorchShallowNeuralClassifier if all you want to do is change the network graph: all you have to do is write a new define_graph. If your graph has new arguments that the user might want to set, then you should also redefine __init__ so that these values are accepted and set as attributes.

For this question, please subclass TorchShallowNeuralClassifier so that it defines the following graph:

$$\begin{align} h_{1} &= xW_{1} + b_{1} \\ r_{1} &= \textbf{Bernoulli}(1 - \textbf{dropout\_prob}, n) \\ d_{1} &= r_1 * h_{1} \\ h_{2} &= f(d_{1}) \\ h_{3} &= h_{2}W_{2} + b_{2} \end{align}$$

Here, $r_{1}$ and $d_{1}$ define a dropout layer: $r_{1}$ is a random binary vector of dimension $n$, where the probability of a value being $1$ is given by $1 - \textbf{dropout_prob}$. $r_{1}$ is multiplied element-wise by our first hidden representation, thereby zeroing out some of the values. The result is fed to the user's activation function $f$, and the result of that is fed through another linear layer to produce $h_{3}$. (Inside TorchShallowNeuralClassifier, $h_{3}$ is the basis for a softmax classifier, so no activation function is applied to it.)

For your implementation, please use nn.Sequential, nn.Linear, and nn.Dropout to define the required layers.

For comparison, using this notation, TorchShallowNeuralClassifier defines the following graph:

$$\begin{align} h_{1} &= xW_{1} + b_{1} \\ h_{2} &= f(h_{1}) \\ h_{3} &= h_{2}W_{2} + b_{2} \end{align}$$

The following code starts this sub-class for you, so that you can concentrate on define_graph. Be sure to make use of self.dropout_prob

For this problem, submit just your completed TorchDeepNeuralClassifier. You needn't evaluate it, though we assume you will be keen to do that!

You can use test_TorchDeepNeuralClassifier to ensure that your network has the intended structure.

Your original system [3 points]

This is a simple dataset, but our focus on the 'word_disjoint' condition ensures that it's a challenging one, and there are lots of modeling strategies one might adopt.

You are free to do whatever you like. We require only that your system differ in some way from those defined in the preceding questions. They don't have to be completely different, though. For example, you might want to stick with the model but represent examples differently, or the reverse.

Keep in mind that, for the bake-off evaluation, the 'edge_disjoint' portions of the data are off limits. You can, though, train on the combination of the 'word_disjoint' 'train' and 'dev' portions. You are free to use different pretrained word vectors and the like. Please do not introduce additional entailment datasets into your training data, though.

Please embed your code in this notebook so that we can rerun it.

In the cell below, please provide a brief technical description of your original system, so that the teaching team can gain an understanding of what it does. This will help us to understand your code and analyze all the submissions to identify patterns and strategies.

Bake-off [1 point]

The goal of the bake-off is to achieve the highest macro-average F1 score on word_disjoint, on a test set that we will make available at the start of the bake-off. The announcement will go out on the discussion forum. To enter, you'll be asked to run nli.bake_off_evaluation on the output of your chosen nli.wordentail_experiment run.

The cells below this one constitute your bake-off entry.

The rules described in the Your original system homework question are also in effect for the bake-off.

Systems that enter will receive the additional homework point, and systems that achieve the top score will receive an additional 0.5 points. We will test the top-performing systems ourselves, and only systems for which we can reproduce the reported results will win the extra 0.5 points.

Late entries will be accepted, but they cannot earn the extra 0.5 points. Similarly, you cannot win the bake-off unless your homework is submitted on time.

The announcement will include the details on where to submit your entry.