DOCSLIB.ORG

PERL: Pivot-Based Domain Adaptation for Pre-Trained Deep Contextualized Embedding Models

Total Page：16

File Type：pdf, Size：1020Kb

Download full-text PDF Read full-text
PERL: Pivot-Based Domain Adaptation for Pre-Trained Deep Contextualized Embedding Models PERL: Pivot-based Domain Adaptation for Pre-trained Deep Contextualized Embedding Models Eyal Ben-David ∗ Carmel Rabinovitz ∗ Roi Reichart Technion, Israel Institute of Technology {[email protected]@campus.jroiri@}technion.ac.il Abstract et al., 2010; Jiang and Zhai, 2007; McClosky et al., 2010; Rush et al., 2012; Schnabel and Schütze, Pivot-based neural representation models 2014). Our focus in this paper is on unsuper- have lead to significant progress in do- vised DA, the setup we consider most realistic. In main adaptation for NLP. However, previous this setup labeled data is available only from the works that follow this approach utilize only labeled data from the source domain and un- source domain while unlabeled data is available labeled data from the source and target do- from both the source and the target domains. mains, but neglect to incorporate massive While various approaches for DA have been unlabeled corpora that are not necessarily proposed (§2), with the prominence of deep neu- drawn from these domains. To alleviate this, ral network (DNN) modeling, attention has been we propose PERL: A representation learn- recently focused on representation learning ap- ing model that extends contextualized word embedding models such as BERT (Devlin proaches. Within representation learning for un- et al., 2019) with pivot-based fine-tuning. supervised DA, two approaches have been shown PERL outperforms strong baselines across particularly useful. In one line of work, DNN- 22 sentiment classification domain adap- based methods which employ compress-based tation setups, improves in-domain model noise reduction to learn cross-domain features performance, yields effective reduced-size have been developed (Glorot et al., 2011; Chen 12 models and increases model stability. et al., 2012). In another line of work, methods based on the distinction between pivot and non- 1 Introduction pivot features (Blitzer et al., 2006, 2007) learn a joint feature representation for the source and Natural Language Processing (NLP) algorithms the target domains. Later on, Ziser and Reichart are constantly improving, gradually approaching (2017, 2018), and Li et al.(2018) married the human level performance (Dozat and Manning, two approaches and achieved substantial improve- 2017; Edunov et al., 2018; Radford et al., 2018). ments on a variety of DA setups. However, those algorithms often depend on the Despite their success, pivot-based DNN mod- availability of large amounts of manually anno- els still only utilize labeled data from the source tated data from the domain where the task is per- domain and unlabeled data from both the source formed. Unfortunately, collecting such annotated and the target domains, but neglect to incorporate data is often costly and laborious, which substan- arXiv:2006.09075v1 [cs.CL] 16 Jun 2020 massive unlabeled corpora that are not necessarily tially limits the applicability of NLP technology. drawn from these domains. With the recent game- Domain Adaptation (DA), training an algorithm changing success of contextualized word embed- on annotated data from a source domain so that ding models trained on such massive corpora (De- it can be effectively applied to other target do- vlin et al., 2019; Peters et al., 2018), it is natu- mains, is one of the ways to solve the above bot- ral to ask whether information from such corpora tleneck. Indeed, over the years substantial efforts can enhance these DA methods, particularly that have been devoted to the DA challenge (Roark and background knowledge from non-contextualized Bacchiani, 2003; III and Marcu, 2006; Ben-David embeddings has shown useful for DA (Plank and ∗ * Both authors equally contributed to this work. Moschitti, 2013; Nguyen et al., 2015). 1Our code is at https://github.com/eyalbd2/ PERL. In this paper we hence propose an unsupervised 2This paper was accepted to TACL in June 2020 DA approach that extends leading approaches based on DNNs and pivot-based ideas, so that 2 Background and Previous Work they can incorporate information encoded in mas- sive corpora (§3). Our model, named PERL: There are several approaches to DA, including in- Pivot-based Encoder Representation of Language, stance re-weighting (Sugiyama et al., 2007; Huang builds on massively pre-trained contextualized et al., 2006; Mansour et al., 2008), sub-sampling word embedding models such as BERT (Devlin from the participating domains (Chen et al., 2011) et al., 2019). To adjust the representations learned and DA through representation learning, where a by these models so that they close the gap be- joint representation is learned based on texts from tween the source and target domains, we fine- the source and target domains (Blitzer et al., 2007; tune their parameters using a pivot-based variant Xue et al., 2008; Ziser and Reichart, 2017, 2018). of the Masked Language Modeling (MLM) objec- We first describe the unsupervised DA pipeline, tive, optimized on unlabeled data from both the continue with representation learning methods for source and the target domains. We further present DA with a focus on pivot-based methods, and, fi- R-PERL (regularized PERL) which facilitates pa- nally, describe contextualized embedding models. rameter sharing for pivots with similar meaning. Unsupervised Domain Adaptation through Representation Learning As said in §1 our fo- We perform extensive experimentation in vari- cus in this work is on unsupervised DA through ous unsupervised DA setups of the task of binary representation learning. A common pipeline for sentiment classification (§4,5). First, for com- this setup consists of two steps: (A) Learning a patibility with previous work, we experiment with representation model (often referred to as the en- the legacy product review domains of Blitzer et al. coder) using the source and target unlabeled data; (2007) (12 setups). We then experiment with more and (B) Training a supervised classifier on the challenging setups, adapting between the above source domain labeled data. To facilitate domain domains and the airline review domain (Nguyen, adaptation, every text fed to the classifier in the 2015) used in Ziser and Reichart(2018) (4 se- second step is first represented by the pre-trained tups), as well as the IMDB movie review domain encoder. This is performed both when the classi- (Maas et al., 2011) (6 setups). We compare PERL fier is trained in the source domain and when it is to the best performing pivot-based methods (Ziser applied to new text from the target domain. and Reichart, 2018; Li et al., 2018) and to DA Exceptions to this pipeline are end-to-end mod- approaches that fine-tune a massively pre-trained els that jointly learn to perform the cross-domain BERT model by optimizing its standard MLM ob- text representation and the classification task. This jective using target-domain unlabeled data (Lee is achieved by training a unified objective on the et al., 2020; Han and Eisenstein, 2019). PERL source domain labeled data and the unlabeled data and R-PERL substantially outperform these base- from both the source and the target. Among these lines, emphasizing the additive effect of massive models are domain adversarial networks (Ganin pre-training and pivot-based fine-tuning. et al., 2016), which were strongly outperformed by Ziser and Reichart(2018) to which we com- pare our methods, and the hierarchical attention As an additional contribution, we show that transfer network (HATN, (Li et al., 2018)), which pivot-based learning is effective beyond improv- is one of our baselines (see below). ing domain adaptation accuracy. Particularly, we Unsupervised DA through representation learn- show that an in-domain variant of PERL substan- ing has followed two main avenues. The first av- tially improves the in-domain performance of a enue consists of works that aim to explicitly build BERT-based sentiment classifier, for varying train- a feature representation that bridges the gap be- ing set sizes (from 100 to 20K labeled examples). tween the domains. A seminal framework in this We also show that PERL facilitates the generation line is structural correspondence learning (SCL, of effective reduced-size DA models. Finally, we (Blitzer et al., 2006, 2007)), that splits the fea- perform an extensive ablation study (§6) that un- ture space into pivot and non-pivot features. A covers PERL’s crucial design choices and demon- large number of works have followed this idea strates the stability of PERL to hyper-parameter (e.g. (Pan et al., 2010; Gouws et al., 2012; Bol- selection compared to other DA methods. legala et al., 2015; Yu and Jiang, 2016; Li et al., 2017, 2018; Tu and Wang, 2019; Ziser and Re- plied to the target domain, PBLM is employed as a ichart, 2017, 2018)) and we discuss it below. contextualized word embedding layer. Notice that Works in the second avenue learn cross-domain PBLM is not pre-trained on massive out of (source representations by training autoencoders (AEs) on and target) domain corpora, and its single-layer, the unlabeled data from the source and target do- unidirectional LSTM architecture is probably not mains. This way they hope to get a more robust ideal for knowledge encoding from such corpora. representation, which is hopefully better suited for Another work in this line is HATN (Li et al., DA. Examples for such models include the stacked 2018). This model automatically learns the denoising AE (SDA, (Vincent et al., 2008; Glorot pivot/non-pivot distinction, rather than following et al., 2011), the marginalized SDA and its variants the SCL definition as Ziser and Reichart(2017, (MSDA, (Chen et al., 2012; Yang and Eisenstein, 2018) did. HATN consists of two hierarchical at- 2014; Clinchant et al., 2016)) and variational AE tention networks, P-net and NP-net. First, it trains based models (Louizos et al., 2016). the P-net on the source labeled data. Then, it de- Recently, Ziser and Reichart(2017, 2018) and codes the most prominent tokens of P-net (i.e.
Load more

Recommended publications
  • Evaluation of Machine Learning Algorithms for Sms Spam Filtering
    EVALUATION OF MACHINE LEARNING ALGORITHMS FOR SMS SPAM FILTERING David Bäckman Bachelor Thesis, 15 credits Bachelor Of Science Programme in Computing Science 2019 Abstract The purpose of this thesis is to evaluate dierent machine learning algorithms and methods for text representation in order to determine what is best suited to use to distinguish between spam SMS and legitimate SMS. A data set that contains 5573 real SMS has been used to train the algorithms K-Nearest Neighbor, Support Vector Machine, Naive Bayes and Logistic Regression. The dierent methods that have been used to represent text are Bag of Words, Bigram and Word2Vec. In particular, it has been investigated if semantic text representations can improve the performance of classication. A total of 12 combinations have been evaluated with help of the metrics accuracy and F1-score. The results shows that Logistic Regression together with Bag of Words reach the highest accuracy and F1-score. Bigram as text representation seems to work worse then the others methods. Word2Vec can increase the performnce for K- Nearst Neigbor but not for the other algorithms. Acknowledgements I would like to thank my supervisor Kai-Florian Richter for all good advice and guidance through the project. I would also like to thank all my classmates for help and support during the education, you have made it possible for me to reach this day. Contents 1 Introduction 1 1.1 Background1 1.2 Purpose and Research Questions1 2 Related Work 3 3 Theoretical Background 5 3.1 The Choice of Algorithms5 3.2 Classication
    [Show full text]
  • NLP - Assignment 2
    NLP - Assignment 2 Week 2 December 27th, 2016 1. A 5-gram model is a order Markov Model: (a) Six (b) Five (c) Four (d) Constant Ans : c) Four 2. For the following corpus C1 of 3 sentences, what is the total count of unique bi- grams for which the likelihood will be estimated? Assume we do not perform any pre-processing, and we are using the corpus as given. (i) ice cream tastes better than any other food (ii) ice cream is generally served after the meal (iii) many of us have happy childhood memories linked to ice cream (a) 22 (b) 27 (c) 30 (d) 34 Ans : b) 27 3. Arrange the words \curry, oil and tea" in descending order, based on the frequency of their occurrence in the Google Books n-grams. The Google Books n-gram viewer is available at https://books.google.com/ngrams: (a) tea, oil, curry (c) curry, tea, oil (b) curry, oil, tea (d) oil, tea, curry Ans: d) oil, tea, curry 4. Given a corpus C2, The Maximum Likelihood Estimation (MLE) for the bigram \ice cream" is 0.4 and the count of occurrence of the word \ice" is 310. The likelihood of \ice cream" after applying add-one smoothing is 0:025, for the same corpus C2. What is the vocabulary size of C2: 1 (a) 4390 (b) 4690 (c) 5270 (d) 5550 Ans: b)4690 The Questions from 5 to 10 require you to analyse the data given in the corpus C3, using a programming language of your choice.
    [Show full text]
  • 3 Dictionaries and Tolerant Retrieval
    Online edition (c)2009 Cambridge UP DRAFT! © April 1, 2009 Cambridge University Press. Feedback welcome. 49 Dictionaries and tolerant 3 retrieval In Chapters 1 and 2 we developed the ideas underlying inverted indexes for handling Boolean and proximity queries. Here, we develop techniques that are robust to typographical errors in the query, as well as alternative spellings. In Section 3.1 we develop data structures that help the search for terms in the vocabulary in an inverted index. In Section 3.2 we study WILDCARD QUERY the idea of a wildcard query: a query such as *a*e*i*o*u*, which seeks doc- uments containing any term that includes all the five vowels in sequence. The * symbol indicates any (possibly empty) string of characters. Users pose such queries to a search engine when they are uncertain about how to spell a query term, or seek documents containing variants of a query term; for in- stance, the query automat* would seek documents containing any of the terms automatic, automation and automated. We then turn to other forms of imprecisely posed queries, focusing on spelling errors in Section 3.3. Users make spelling errors either by accident, or because the term they are searching for (e.g., Herman) has no unambiguous spelling in the collection. We detail a number of techniques for correcting spelling errors in queries, one term at a time as well as for an entire string of query terms. Finally, in Section 3.4 we study a method for seeking vo- cabulary terms that are phonetically close to the query term(s).
    [Show full text]
  • N-Gram-Based Machine Translation
    N-gram-based Machine Translation ∗ JoseB.Mari´ no˜ ∗ Rafael E. Banchs ∗ Josep M. Crego ∗ Adria` de Gispert ∗ Patrik Lambert ∗ Jose´ A. R. Fonollosa ∗ Marta R. Costa-jussa` Universitat Politecnica` de Catalunya This article describes in detail an n-gram approach to statistical machine translation. This ap- proach consists of a log-linear combination of a translation model based on n-grams of bilingual units, which are referred to as tuples, along with four specific feature functions. Translation performance, which happens to be in the state of the art, is demonstrated with Spanish-to-English and English-to-Spanish translations of the European Parliament Plenary Sessions (EPPS). 1. Introduction The beginnings of statistical machine translation (SMT) can be traced back to the early fifties, closely related to the ideas from which information theory arose (Shannon and Weaver 1949) and inspired by works on cryptography (Shannon 1949, 1951) during World War II. According to this view, machine translation was conceived as the problem of finding a sentence by decoding a given “encrypted” version of it (Weaver 1955). Although the idea seemed very feasible, enthusiasm faded shortly afterward because of the computational limitations of the time (Hutchins 1986). Finally, during the nineties, two factors made it possible for SMT to become an actual and practical technology: first, significant increment in both the computational power and storage capacity of computers, and second, the availability of large volumes of bilingual data. The first SMT systems were developed in the early nineties (Brown et al. 1990, 1993). These systems were based on the so-called noisy channel approach, which models the probability of a target language sentence T given a source language sentence S as the product of a translation-model probability p(S|T), which accounts for adequacy of trans- lation contents, times a target language probability p(T), which accounts for fluency of target constructions.
    [Show full text]
  • Title and Link Description Improving Distributional Similarity With
    title and link description Improving Distributional Similarity • word similarity with Lessons Learned from Word ... • analogy • qualitativ: compare neighborhoods across Dependency-Based Word Embeddings embeddings • word similarity • word similarity Multi-Granularity Chinese Word • analogy Embedding • qualitative: local neighborhoods A Mixture Model for Learning Multi- • word similarity Sense Word Embeddings • analogy Learning Crosslingual Word • multilingual Embeddings without Bilingual Corpora • word similarities (monolingual) Word Embeddings with Limited • word similarity Memory • phrase similarity A Latent Variable Model Approach to • analogy PMI-based Word Embeddings Prepositional Phrase Attachment over • extrinsic Word Embedding Products A Simple Word Embedding Model for • lexical substitution (nearest Lexical Substitution neighbors after vector averaging) Segmentation-Free Word Embedding • extrinsic for Unsegmented Languages • word similarity Evaluation methods for unsupervised • analogy word embeddings • concept categorization • selectional preference Dict2vec : Learning Word Embeddings • word similarity using Lexical Dictionaries • extrinsic Task-Oriented Learning of Word • extrinsic Embeddings for Semantic Relation ... Refining Word Embeddings for • extrinsic Sentiment Analysis Language classification from bilingual • word similarity word embedding graphs Learning principled bilingual mappings • multilingual of word embeddings while ... A Word Embedding Approach to • extrinsic Identifying Verb-Noun Idiomatic ... Deep Multilingual
    [Show full text]
  • Arxiv:2006.14799V2 [Cs.CL] 18 May 2021 the Same User Input
    Evaluation of Text Generation: A Survey Evaluation of Text Generation: A Survey Asli Celikyilmaz [email protected] Facebook AI Research Elizabeth Clark [email protected] University of Washington Jianfeng Gao [email protected] Microsoft Research Abstract The paper surveys evaluation methods of natural language generation (NLG) systems that have been developed in the last few years. We group NLG evaluation methods into three categories: (1) human-centric evaluation metrics, (2) automatic metrics that require no training, and (3) machine-learned metrics. For each category, we discuss the progress that has been made and the challenges still being faced, with a focus on the evaluation of recently proposed NLG tasks and neural NLG models. We then present two examples for task-specific NLG evaluations for automatic text summarization and long text generation, and conclude the paper by proposing future research directions.1 1. Introduction Natural language generation (NLG), a sub-field of natural language processing (NLP), deals with building software systems that can produce coherent and readable text (Reiter & Dale, 2000a) NLG is commonly considered a general term which encompasses a wide range of tasks that take a form of input (e.g., a structured input like a dataset or a table, a natural language prompt or even an image) and output a sequence of text that is coherent and understandable by humans. Hence, the field of NLG can be applied to a broad range of NLP tasks, such as generating responses to user questions in a chatbot, translating a sentence or a document from one language into another, offering suggestions to help write a story, or generating summaries of time-intensive data analysis.
    [Show full text]
  • Word Senses and Wordnet Lady Bracknell
    Speech and Language Processing. Daniel Jurafsky & James H. Martin. Copyright © 2021. All rights reserved. Draft of September 21, 2021. CHAPTER 18 Word Senses and WordNet Lady Bracknell. Are your parents living? Jack. I have lost both my parents. Lady Bracknell. To lose one parent, Mr. Worthing, may be regarded as a misfortune; to lose both looks like carelessness. Oscar Wilde, The Importance of Being Earnest ambiguous Words are ambiguous: the same word can be used to mean different things. In Chapter 6 we saw that the word “mouse” has (at least) two meanings: (1) a small rodent, or (2) a hand-operated device to control a cursor. The word “bank” can mean: (1) a financial institution or (2) a sloping mound. In the quote above from his play The Importance of Being Earnest, Oscar Wilde plays with two meanings of “lose” (to misplace an object, and to suffer the death of a close person). We say that the words ‘mouse’ or ‘bank’ are polysemous (from Greek ‘having word sense many senses’, poly- ‘many’ + sema, ‘sign, mark’).1 A sense (or word sense) is a discrete representation of one aspect of the meaning of a word. In this chapter WordNet we discuss word senses in more detail and introduce WordNet, a large online the- saurus —a database that represents word senses—with versions in many languages. WordNet also represents relations between senses. For example, there is an IS-A relation between dog and mammal (a dog is a kind of mammal) and a part-whole relation between engine and car (an engine is a part of a car).
    [Show full text]
  • A Unigram Orientation Model for Statistical Machine Translation
    A Unigram Orientation Model for Statistical Machine Translation Christoph Tillmann IBM T.J. Watson Research Center Yorktown Heights, NY 10598 [email protected] Abstract b5 airspace In this paper, we present a unigram segmen- b4 tation model for statistical machine transla- Lebanese b tion where the segmentation units are blocks: 3 pairs of phrases without internal structure. The violate b2 segmentation model uses a novel orientation warplanes component to handle swapping of neighbor b1 blocks. During training, we collect block un- Israeli igram counts with orientation: we count how A A A t A A A often a block occurs to the left or to the right of l l l n l l l some predecessor block. The orientation model T H A t m j l is shown to improve translation performance A r s h j w b } over two models: 1) no block re-ordering is b r k A y n r y A l A used, and 2) the block swapping is controlled A P } n only by a language model. We show exper- t y y imental results on a standard Arabic-English l y translation task. P 1 Introduction Figure 1: An Arabic-English block translation example taken from the devtest set. The Arabic words are roman- In recent years, phrase-based systems for statistical ma- ized. chine translation (Och et al., 1999; Koehn et al., 2003; Venugopal et al., 2003) have delivered state-of-the-art performance on standard translation tasks. In this pa- section 3, block swapping where only a trigram language per, we present a phrase-based unigram system similar model is used to compute probabilities between neighbor to the one in (Tillmann and Xia, 2003), which is ex- blocks fails to improve translation performance.
    [Show full text]
  • Heafield, K. and A. Lavie. "Voting on N-Grams for Machine Translation
    Voting on N-grams for Machine Translation System Combination Kenneth Heafield and Alon Lavie Language Technologies Institute Carnegie Mellon University 5000 Forbes Avenue Pittsburgh, PA 15213 fheafield,[email protected] Abstract by a single weight and further current features only guide decisions at the word level. To remedy this System combination exploits differences be- situation, we propose new features that account for tween machine translation systems to form a multiword behavior, each with a separate set of sys- combined translation from several system out- tem weights. puts. Core to this process are features that reward n-gram matches between a candidate 2 Features combination and each system output. Systems differ in performance at the n-gram level de- Most combination schemes generate many hypothe- spite similar overall scores. We therefore ad- sis combinations, score them using a battery of fea- vocate a new feature formulation: for each tures, and search for a hypothesis with highest score system and each small n, a feature counts to output. Formally, the system generates hypoth- n-gram matches between the system and can- esis h, evaluates feature function f, and multiplies didate. We show post-evaluation improvement of 6.67 BLEU over the best system on NIST linear weight vector λ by feature vector f(h) to ob- T MT09 Arabic-English test data. Compared to tain score λ f(h). The score is used to rank fi- a baseline system combination scheme from nal hypotheses and to prune partial hypotheses dur- WMT 2009, we show improvement in the ing beam search. The beams contain hypotheses of range of 1 BLEU point.
    [Show full text]
  • Part I Word Vectors I: Introduction, Svd and Word2vec 2 Natural Language in Order to Perform Some Task
    CS224n: Natural Language Processing with Deep 1 Learning 1 Course Instructors: Christopher Lecture Notes: Part I Manning, Richard Socher 2 Word Vectors I: Introduction, SVD and Word2Vec 2 Authors: Francois Chaubard, Michael Fang, Guillaume Genthial, Rohit Winter 2019 Mundra, Richard Socher Keyphrases: Natural Language Processing. Word Vectors. Singu- lar Value Decomposition. Skip-gram. Continuous Bag of Words (CBOW). Negative Sampling. Hierarchical Softmax. Word2Vec. This set of notes begins by introducing the concept of Natural Language Processing (NLP) and the problems NLP faces today. We then move forward to discuss the concept of representing words as numeric vectors. Lastly, we discuss popular approaches to designing word vectors. 1 Introduction to Natural Language Processing We begin with a general discussion of what is NLP. 1.1 What is so special about NLP? What’s so special about human (natural) language? Human language is a system specifically constructed to convey meaning, and is not produced by a physical manifestation of any kind. In that way, it is very different from vision or any other machine learning task. Natural language is a dis- Most words are just symbols for an extra-linguistic entity : the crete/symbolic/categorical system word is a signifier that maps to a signified (idea or thing). For instance, the word "rocket" refers to the concept of a rocket, and by extension can designate an instance of a rocket. There are some exceptions, when we use words and letters for expressive sig- naling, like in "Whooompaa". On top of this, the symbols of language can be encoded in several modalities : voice, gesture, writing, etc that are transmitted via continuous signals to the brain, which itself appears to encode things in a continuous manner.
    [Show full text]
  • Phrase Based Language Model for Statistical Machine Translation
    Phrase Based Language Model for Statistical Machine Translation Technical Report by Chen, Geliang 1 Abstract Reordering is a challenge to machine translation (MT) systems. In MT, the widely used approach is to apply word based language model (LM) which considers the constituent units of a sentence as words. In speech recognition (SR), some phrase based LM have been proposed. However, those LMs are not necessarily suitable or optimal for reordering. We propose two phrase based LMs which considers the constituent units of a sentence as phrases. Experiments show that our phrase based LMs outperform the word based LM with the respect of perplexity and n-best list re-ranking. Key words: machine translation, language model, phrase based This version of report is identical to the dissertation approved from Yuanpei Institute of Peking University to receive the Bachelor Degree of Probability and Statistics by Chen, Geliang. Advisor: Assistant Professor Dr.rer.nat. Jia Xu Evaluation Date: 20. June. 2013 2 Contents Abstract .......................................................................................................................... 2 Contents …………………………………………………………………………………………………………………..3 1 Introduction ................................................................................................................. 4 2 Review of the Word Based LM ................................................................................... 5 2.1 Sentence probability ............................................................................................
    [Show full text]
  • Phrase-Based Attentions
    Under review as a conference paper at ICLR 2019 PHRASE-BASED ATTENTIONS Phi Xuan Nguyen, Shafiq Joty Nanyang Technological University, Singapore [email protected], [email protected] ABSTRACT Most state-of-the-art neural machine translation systems, despite being different in architectural skeletons (e.g., recurrence, convolutional), share an indispensable feature: the Attention. However, most existing attention methods are token-based and ignore the importance of phrasal alignments, the key ingredient for the suc- cess of phrase-based statistical machine translation. In this paper, we propose novel phrase-based attention methods to model n-grams of tokens as attention entities. We incorporate our phrase-based attentions into the recently proposed Transformer network, and demonstrate that our approach yields improvements of 1.3 BLEU for English-to-German and 0.5 BLEU for German-to-English transla- tion tasks on WMT newstest2014 using WMT’16 training data. 1 INTRODUCTION Neural Machine Translation (NMT) has established breakthroughs in many different translation tasks, and has quickly become the standard approach to machine translation. NMT offers a sim- ple encoder-decoder architecture that is trained end-to-end. Most NMT models (except a few like (Kaiser & Bengio, 2016) and (Huang et al., 2018)) possess attention mechanisms to perform align- ments of the target tokens to the source tokens. The attention module plays a role analogous to the word alignment model in Statistical Machine Translation or SMT (Koehn, 2010). In fact, the trans- former network introduced recently by Vaswani et al. (2017) achieves state-of-the-art performance in both speed and BLEU scores (Papineni et al., 2002) by using only attention modules.
    [Show full text]