Fully Non-autoregressive Neural Machine Translation

This page mainly includes instructions for reproducing results from the following papers

• fully non-autoregressive neural machine translation: tricks of the trade (Gu et al., 2020).

Installation

This code is based on an old version of Fairseq, which has changed significantly after the above paper was published. We previously found it was not easy to merge our code to the up-to-date fairseq version, so we decided to open-source with a saperate code.

To start with,

git clone --recursive git@github.com:shawnkx/Fully-NAT.git

(Optional) ctcdecode is used for advanced decoding with beam-search and language model. Installation follows the following instruction

cd ctcdecode; pip install .

Then,

cd fairseq-stable
python setup.py build_ext --inplace

This code is tested on pytorch==1.7.1, cuda 10.2.

Dataset

We follow the instructions here. You can also download the preprocessed datasets as follows:

Direction	Download
Ro <-> En	download (.zip)
En <-> De	download (.zip)
Ja --> En	download (.zip)

Pre-trained Models

Ro-En

Direction	Model	BLEU		Direction	Model	BLEU
Ro -> En	roen.distill.ctc	34.07		En -> Ro	enro.distill.ctc	33.41
Ro -> En	roen.distill.ctc.glat	34.16		En -> Ro	enro.distill.ctc.glat	33.71
Ro -> En	roen.distill.ctc.vae	33.87		En -> Ro	enro.distill.ctc.vae	33.79

En-De

Direction	Model	BLEU		Direction	Model	BLEU
De -> En	deen.distill.ctc	30.46		En -> De	ende.distill.ctc	26.51
De -> En	deen.distill.ctc.glat	31.37		En -> De	ende.distill.ctc.glat	27.20
De -> En	deen.distill.ctc.vae	31.10		En -> De	ende.distill.ctc.vae	27.49

Ja-En

Direction	Model	BLEU	BLEU + 4gram-LM + beam20
Ja -> En	jaen.distill.ctc.vae	18.73	21.41 (Download 4-gram En-LM)

Train a model

The following command will train a baseline NAT model with CTC loss.

python train.py ${databin_dir} \
    --fp16 \
    --left-pad-source False --left-pad-target False \
    --arch cmlm_transformer_ctc --task translation_lev \
    --noise 'full_mask' --valid-noise 'full_mask' \
    --dynamic-upsample --src-upsample 3 \
    --decoder-learned-pos --encoder-learned-pos \
    --apply-bert-init --share-all-embeddings \
    --optimizer adam --adam-betas '(0.9, 0.999)' --adam-eps 1e-06 \
    --clip-norm 2.4 --dropout 0.3 --lr-scheduler inverse_sqrt \
    --warmup-init-lr 1e-07 --warmup-updates 10000 --lr 0.0005 --min-lr 1e-09 \
    --criterion nat_loss --predict-target 'all' --loss-type 'ctc' \
    --axe-eps --force-eps-zero \
    --label-smoothing 0.1 --weight-decay 0.01 \
    --max-tokens 4096 --update-freq 1 \
    --max-update 300000 --save-dir ${model_dir} --save-interval-updates 5000 \
    --no-epoch-checkpoints --keep-interval-updates 10 --keep-best-checkpoints 5 \
    --seed 2 --log-interval 100 --no-progress-bar \
    --eval-bleu --eval-bleu-args '{"iter_decode_max_iter":0,"iter_decode_collapse_repetition":true}' \
    --eval-bleu-detok 'space' \
    --eval-tokenized-bleu --eval-bleu-remove-bpe '@@ ' --eval-bleu-print-samples \
    --best-checkpoint-metric bleu --maximize-best-checkpoint-metric \
    --tensorboard-logdir ${ckpt_dir}/tensorboard/${expname} 

Use the --latent-dim 8 flag to add the latent variables (VAEs).
Use --glat-sampling-ratio 0.5 --poisson-mask --replace-target-embed to add the GLAT training.

Translate

For inference,

python fairseq_cli/generate.py ${databin_dir} \
    --task translation_lev \
    --path checkpoints/checkpoint_best.pt \
    --gen-subset test \
    --axe-eps  --iter-decode-collapse-repetition --force-eps-zero \
    --left-pad-source False --left-pad-target False \
    --iter-decode-max-iter 0 --beam 1 \
    --remove-bpe --batch-size 200 \

In the end of the generation, we can see the tokenized BLEU score for the translation. For datasets based on BPEs (Ro-En, En-De), we report standard tokenized BLEU, for datasets using sentencepiece (Ja-En), we report sacre-bleu by replacing --remove-bpe as --remove-bpe sentencepiece --scoring sacrebleu.

Set --batch-size 1 to compute the latency when decoding one sentence at a time.

Advanced Decoding Methods

CTC Beam Search

We can use CTC Beam Search (+ language model) to improve the translation quality

python fairseq_cli/generate.py ${databin_dir} \
    --task translation_lev \
    --path checkpoints/checkpoint_best.pt \
    --gen-subset test \
    --axe-eps  --iter-decode-collapse-repetition --force-eps-zero \
    --left-pad-source False --left-pad-target False \
    --iter-decode-max-iter 0 --beam 1 \
    --remove-bpe sentencepiece --scoring sacrebleu \
    --batch-size 20 \
    --model-overrides "{'use_ctc_beamsearch':True, 'ctc_bs_beam':${ctc_beam_size}, 'ctc_bs_alpha':${alpha}, 'ctc_bs_beta':${beta}, 'ctc_bs_lm_path':'${lm_path}'}"

For example, for Ja-En model, we use alpha=0.4, beta=2.2, ctc_beam_size=20
If ${lm_path} is None, we apply CTC beam search without language model.

Citation

@inproceedings{gu-kong-2021-fully,
    title = "Fully Non-autoregressive Neural Machine Translation: Tricks of the Trade",
    author = "Gu, Jiatao  and
      Kong, Xiang",
    booktitle = "Findings of the Association for Computational Linguistics: ACL-IJCNLP 2021",
    month = aug,
    year = "2021",
    address = "Online",
    publisher = "Association for Computational Linguistics",
    url = "https://aclanthology.org/2021.findings-acl.11",
    doi = "10.18653/v1/2021.findings-acl.11",
    pages = "120--133",
}