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Title: Language Modeling Teaches You More than Translation Does: Lessons Learned Through Auxiliary Task Analysis

Paper content:
There is mounting evidence that pretraining can be valuable for neural network language understanding models, but we do not yet have a clear understanding of how the choice of pretraining objective affects the type of linguistic information that models learn.
With this in mind, we compare four objectives---language modeling, translation, skip-thought, and autoencoding---on their ability to induce syntactic and part-of-speech information, holding constant the genre and quantity of training data.
We find that representations from language models consistently perform best on our syntactic auxiliary prediction tasks, even when trained on relatively small amounts of data, which suggests that language modeling may be the best data-rich pretraining task for transfer learning applications requiring syntactic information.
We also find that a randomly-initialized, frozen model can perform strikingly well on our auxiliary tasks, but that this effect disappears when the amount of training data for the auxiliary tasks is reduced.
Representation learning with deep recurrent neural networks has revolutionized natural language processing and replaced many of the expert-designed, linguistic features previously used.
Recently, researchers have begun to investigate the properties of representations learned by networks by training auxiliary classifiers that use the hidden states of frozen pretrained models to perform other tasks.
These investigations have shown that when deep LSTM RNNs (Hochreiter and Schmidhuber, 1997) are trained on tasks like machine translation, they latently identify substantial syntactic and semantic information about their input sentences, including part-of-speech (Shi et al., 2016; Belinkov et al., 2017a,b; Blevins et al., 2018) .These
intriguing findings lead us to ask the following questions:1. How does
the training task affect how well models latently learn syntactic properties? Which tasks
are better at inducing these properties?2. How does
the
amount of data the model is trained on affect these results? When does training
on more data help?We investigate these
questions by holding the data source and model architecture constant, while varying both the training task and the amount of training data. Specifically, we examine
models trained on English-German (En-De) translation, language modeling, skip-thought (Kiros et al., 2015) , and autoencoding, in addition to an untrained baseline model. We control for the data
domain by exclusively training on datasets from the 2016 Conference on Machine Translation (WMT; Bojar et al., 2016) . We train models on all
tasks using the parallel En-De corpus and a small subset of that corpus, which allows us to make a fair comparison across all five models. Additionally, we augment
the parallel dataset with a large monolingual corpus from WMT to examine how the performance of the unsupervised tasks (all but translation) scale with more data.Throughout our work, we focus on the syntactic evaluation tasks of part-of-speech (POS) tagging and Combinatorial Categorical Grammar (CCG) supertagging. Supertagging is a building
block for parsing as these tags constrain the ways in which words can compose, largely determining the parse of the sentence. CCG supertagging thus allows
us to measure the degree to which models learn syntactic structure above the word. We focus our analysis on representations
learned by language models and by the encoders of sequence-to-sequence models, as translation encoders have been found to learn richer representations of POS and morphological information than translation decoders (Belinkov et al., 2017a) .We find that for POS and CCG tagging, bidirectional
language models (BiLMs)-created by separately training forward and backward language models, and concatenating their hidden statesoutperform models trained on all other tasks. Even BiLMs trained on relatively small amounts of data
(1 million sentences) outperform translation and skip-thought models trained on larger datasets (5 million and 63 million sentences respectively).Our inclusion of an untrained LSTM baseline allows us to
study the effect of training on state representations. We find, surprisingly, that randomly initialized LSTMs underperform
our best trained models by only a few percentage points when we use all of the available labeled data to train classifiers for our auxiliary tasks. When we reduce the amount of classifier training data though, the performance
of the randomly initialized LSTM model drops far below those of trained models. We hypothesize that this occurs because training the classifiers on large amounts
of auxiliary task data allows them to memorize configurations of words seen in the training set and their associated tags. We test this hypothesis by training classifiers to predict the identity of neighboring
words from a given hidden state, and find that randomly initialized models outperform all trained models on this task. Our findings demonstrate that our best trained models do well on the tagging tasks because
they are truly learning representations that conform to our notions of POS and CCG tagging, and not because the classifiers we train are able to recover neighboring word identity information well.
By controlling for the genre and quantity of the training data, we make fair comparisons between several data-rich training tasks in their ability to induce syntactic information.
We find that bidirectional language models (BiLMs) do better than translation and skip-thought encoders at extracting useful features for POS tagging and CCG supertagging.
Moreover, this improvement holds even when the BiLMs are trained on substantially less data than competing models.
Although, due to limited parallel data, we could not compare BiLMs and translation encoders on more than 5 million sentences, our results suggest that for syntactic information, there is no need to compare these two models trained on more data, as BiLMs consistently outperform translation encoders in all data regimes.We also find that randomly initialized encoders extract usable features for POS and CCG tagging, at least when the auxiliary POS and CCG classifiers are themselves trained on reasonably large amounts of data.
However, the performance of untrained models drops sharply relative to trained ones when using smaller amounts of the classifier data.
We investigate further and find that untrained models outperform trained ones on the task of neighboring word identity prediction, which confirms that trained encoders do not perform well on tagging tasks because the classifiers are simply memorizing word identity information.
We also find that both trained and untrained LSTMs store more local neighboring word identity information in lower layers and more distant word identity information in upper layers, which suggests that depth in LSTMs allow them to capture larger context information.Our results suggest that for transfer learning, bidirectional language models like ELMo (Peters et al., 2018) capture more useful features than translation encoders-and that this holds even on genres or languages for which data is not abundant.
However, the scope of our experiments is limited, and we still know little about the representations of models trained on other supervised tasks, or precisely how the choice of training task affects the type of syntactic information that is learned.
Our work also highlights the interesting behavior of randomly initialized LSTMs, which show an ability to preserve the contents of their inputs significantly better than trained models.

assistant: text: Representations from language models consistently perform better than translation encoders on syntactic auxiliary prediction tasks.