Lecture 17 Machine Translation

Machine translation (MT) is the task of translating text from one source language to another target language

• why?
  • Removes language barrier
  • Makes information in any languages accessible to anyone
  • But translation is a classic “AI-hard” challenge
  • Difficult to preserve the meaning and the fluency of the text after translation
• MT is difficult
  • Not just simple word for word translation
  • Structural changes, e.g., syntax and semantics
  • Multiple word translations, idioms(习语，成语，方言)
  • Inflections for gender, case etc
  • Missing information (e.g., determiners)

Statistical MT

• earliy MT
  • Started in early 1950s
  • Motivated by the Cold War to translate Russian to English
  • Rule-based system
    • Use bilingual dictionary to map Russian words to English words
  • Goal: translate 1-2 million words an hour within 5 years
• statistical MT

  • Given French sentence f, aim is to find the best English sentence e

    • $argma{x}_{e}P(e|f)$

  • Use Baye’s rule to decompose into two components

    • $argma{x}_{e}P(f|e)P(e)$

  • language vs translation model

    • $argma{x}_{e}P(f|e)P(e)$
    • $P(e)$: language model
      • learn how to write fluent English text
    • $P(f|e)$: translation model
      • learns how to translate words and phrases from English to French

  • how to learn LM and TM

    • Language model:
      • Text statistics in large monolingual(仅一种语言的) corpora (n-gram models)
    • Translation model:
      • Word co-occurrences in parallel corpora
      • i.e. English-French sentence pairs

  • parallel corpora

    • One text in multiple languages
    • Produced by human translation
      • Bible, news articles, legal transcripts, literature, subtitles
      • Open parallel corpus: http://opus.nlpl.eu/

  • models of translation

    • how to learn $P(f|e)$ from paralell text?
    • We only have sentence pairs; words are not aligned in the parallel text
    • I.e. we don’t have word to word translation

  • alignment

    • Idea: introduce word alignment as a latent variable into the model

      • $P(f,a|e)$

    • Use algorithms such as expectation maximisation (EM) to learn (e.g. GIZA++)

    • complexity

      • some words are dropped and have no alignment

      • One-to-many alignment

      • many-to-one alignment

      • many-to-many alignment

  • summary

    • A very popular field of research in NLP prior to 2010s
    • Lots of feature engineering
    • State-of-the-art systems are very complex
      • Difficult to maintain
      • Significant effort needed for new language pairs

Neural MT

• introduction

  • Neural machine translation is a new approach to do machine translation
  • Use a single neural model to directly translate from source to target
  • from model perspective, a lot simpler
  • from achitecture perspective, easier to maintain
  • Requires parallel text
  • Architecture: encoder-decoder model
    • 1st RNN to encode the source sentence
    • 2nd RNN to decode the target sentence

• neural MT

  • The decoder RNN can be interpreted as a conditional language model

    • Language model: predicts the next word given previous words in target sentence y
    • Conditional: prediction is also conditioned on the source sentence x

  • $P(y|x)=P(y_1|x)P(y_2|y_1,x)...P(y_t|y_1,...,y_{t-1},x)$

  • training

    • Requires parallel corpus just like statistical MT

    • Trains with next word prediction, just like a language model

    • loss

      • During training, we have the target sentence
      • We can therefore feed the right word from target sentence, one step at a time

  • decoding at test time

    • But at test time, we don’t have the target sentence (that’s what we’re trying to predict!)

    • argmax: take the word with the highest probability at every step

    • exposure bias

      • Describes the discrepancy(差异) between training and testing
      • Training: always have the ground truth tokens at each step
      • Test: uses its own prediction at each step
      • Outcome: model is unable to recover from its own error(error propagation)

    • greedy decoding

      • argmax decoding is also called greedy decoding
      • Issue: does not guarantee optimal probability $P(y|x)$

    • exhaustive search decoding

      • To find optimal $P(y|x)$, we need to consider every word at every step to compute the probability of all possible sequences
      • $O(V^n)$ where V = vocab size; n = sentence length
      • Far too expensive to be feasible

    • beam search decoding

      • Instead of considering all possible words at every step, consider k best words
      • That is, we keep track of the top-k words that produce the best partial translations (hypotheses) thus far
      • k = beam width (typically 5 to 10)
      • k = 1 = greedy decoding
      • k = V = exhaustive search decoding
      • example:

    • when to stop

      • When decoding, we stop when we generate token
      • But multiple hypotheses may terminate their sentence at different time steps
      • We store hypotheses that have terminated, and continue explore those that haven’t
      • Typically we also set a maximum sentence length that can be generated (e.g. 50 words)

  • issues of NMT

    • Information of the whole source sentence is represented by a single vector
    • NMT can generate new details not in source sentence
    • NMT tend to generate not very fluent sentences ($\times$, usually fluent, a strength)
    • Black-box model; difficult to explain when it doesn’t work

  • summary

    • Single end-to-end model
      • Statistical MT systems have multiple subcomponents
      • Less feature engineering
      • Can produce new details that are not in the source sentence (hallucination:错觉，幻觉)

Attention Mechanism

• With a long source sentence, the encoded vector is unlikely to capture all the information in the sentence
• This creates an information bottleneck(cannot capture all information in a long sentence in a single short vector)
• attention

  • For the decoder, at every time step allow it to ‘attend’ to words in the source sentence

  • encoder-decoder with attention

  • variants

    • attention
      • dot product: $s_t^T{h}_i$
      • bilinear: $s_t^{T}W{h}_i$
      • additive: v^Ttanh(W_ss_t+W_hh_i)
    • $c_t$ can be injected to the current state ($s_t$), or to the input word ($y_t$)

  • summary

    • Solves the information bottleneck issue by allowing decoder to have access to the source sentence words directly(reduce hallucination a bit, direct access to source words, less likely to generate new words not related to source sentence)
    • Provides some form of interpretability (look at attention distribution to see what source word is attended to)
      • Attention weights can be seen as word alignments
    • Most state-of-the-art NMT systems use attention
      • Google Translate (https://slator.com/technology/google-facebook-amazonneural-machine-translation-just-had-its-busiest-month-ever/)

Evaluation

• MT evaluation
  • BLEU: compute n-gram overlap between “reference” translation(ground truth) and generated translation
  • Typically computed for 1 to 4-gram
    • $BLEU=BP\times exp(\frac{1}{N}{\sum }_n^{N}log{p}_n)$, where BP $\to$ “Brevity Penalty” to penalise short outputs
    • $p_n=\frac{\#correctn-grams}{\#predictedn-grams}$
    • $BP=min(1,\frac{outputlength}{referencelength})$

Conclusion

• Statistical MT
• Neural MT
  • Nowadays use Transformers rather than RNNs
• Encoder-decoder with attention architecture is a general architecture that can be used for other tasks
  • Summarisation (lecture 21)
  • Other generation tasks such as dialogue generation