Levenshtein Transformer paper code

Summary:
Code for our NeurIPS paper [Levenshtein Transformer](https://arxiv.org/abs/1905.11006)
* Added Levenshtein Transformer model, task and criterion class
* Added iterative NAT Transformer, insertion Transformer and CMLM Transformer model class for baselines
* Add an option for prepending BOS to dictionary class and translation task class

Reviewed By: myleott

Differential Revision: D17297372

fbshipit-source-id: 54eca60831ae95dc721c2c34e882e1810ee575c7

Author: kahne

README.md

@@ -6,6 +6,7 @@ modeling and other text generation tasks.
 
 ### What's New:
 
+- September 2019: [Nonautoregressive translation code released](examples/nonautoregressive_translation/README.md)
 - August 2019: [WMT'19 models released](examples/wmt19/README.md)
 - July 2019: fairseq relicensed under MIT license
 - July 2019: [RoBERTa models and code released](examples/roberta/README.md)
@@ -32,6 +33,13 @@ Fairseq provides reference implementations of various sequence-to-sequence model
   - [Mixture Models for Diverse Machine Translation: Tricks of the Trade (Shen et al., 2019)](examples/translation_moe/README.md)
   - [RoBERTa: A Robustly Optimized BERT Pretraining Approach (Liu et al., 2019)](examples/roberta/README.md)
   - [Facebook FAIR's WMT19 News Translation Task Submission (Ng et al., 2019)](examples/wmt19/README.md)
+- **Non-autoregressive Transformers**
+  - Non-Autoregressive Neural Machine Translation (Gu et al., 2017)
+  - Deterministic Non-Autoregressive Neural Sequence Modeling by Iterative Refinement (Lee et al. 2018)
+  - Insertion Transformer: Flexible Sequence Generation via Insertion Operations (Stern et al. 2019)
+  - Mask-Predict: Parallel Decoding of Conditional Masked Language Models (Ghazvininejad et al., 2019)
+  - [Levenshtein Transformer (Gu et al., 2019)](examples/nonautoregressive_translation/README.md)
+
 
 **Additionally:**
 - multi-GPU (distributed) training on one machine or across multiple machines
@@ -50,7 +58,7 @@ translation and language modeling datasets.
 
 # Requirements and Installation
 
-* [PyTorch](http://pytorch.org/) version >= 1.1.0
+* [PyTorch](http://pytorch.org/) version >= 1.2.0
 * Python version >= 3.5
 * For training new models, you'll also need an NVIDIA GPU and [NCCL](https://github.com/NVIDIA/nccl)
 * **For faster training** install NVIDIA's [apex](https://github.com/NVIDIA/apex) library with the `--cuda_ext` option
@@ -92,6 +100,7 @@ as well as example training and evaluation commands.
 - [Language Modeling](examples/language_model/README.md): convolutional and transformer models are available
 
 We also have more detailed READMEs to reproduce results from specific papers:
+- [Levenshtein Transformer (Gu et al., 2019)](examples/nonautoregressive_translation/README.md)
 - [Facebook FAIR's WMT19 News Translation Task Submission (Ng et al., 2019)](examples/wmt19/README.md)
 - [RoBERTa: A Robustly Optimized BERT Pretraining Approach (Liu et al., 2019)](examples/roberta/README.md)
 - [wav2vec: Unsupervised Pre-training for Speech Recognition (Schneider et al., 2019)](examples/wav2vec/README.md)

examples/nonautoregressive_translation/README.md

@@ -0,0 +1,90 @@
+# Non-autoregressive Neural Machine Translation (NAT)
+
+This page mainly includes instructions for reproducing results from the paper
+* [Levenshtein Transformer (Gu et al., 2019)](https://arxiv.org/abs/1905.11006).
+
+We also provided our own implementations for several popular non-autoregressive-based models as reference:<br>
+* [Non-Autoregressive Neural Machine Translation (Gu et al., 2017)](https://arxiv.org/abs/1711.02281)<br>
+* [Deterministic Non-Autoregressive Neural Sequence Modeling by Iterative Refinement (Lee et al. 2018)](https://arxiv.org/abs/1802.06901)<br>
+* [Insertion Transformer: Flexible Sequence Generation via Insertion Operations (Stern et al. 2019)](https://arxiv.org/abs/1902.03249)<br>
+* [Mask-Predict: Parallel Decoding of Conditional Masked Language Models (Ghazvininejad et al., 2019)](https://arxiv.org/abs/1904.09324v2)
+
+## Dataset
+
+First, follow the [instructions to download and preprocess the WMT'14 En-De dataset](../translation#prepare-wmt14en2desh).
+Make sure to learn a joint vocabulary by passing the `--joined-dictionary` option to `fairseq-preprocess`.
+
+### Knowledge Distillation
+Following [Gu et al. 2019](https://arxiv.org/abs/1905.11006), [knowledge distillation](https://arxiv.org/abs/1606.07947) from an autoregressive model can effectively simplify the training data distribution, which is sometimes essential for NAT-based models to learn good translations.
+The easiest way of performing distillation is to follow the [instructions of training a standard transformer model](../translation) on the same data, and then decode the training set to produce a distillation dataset for NAT.
+
+### Download
+We also provided the preprocessed [original](http://dl.fbaipublicfiles.com/nat/original_dataset.zip) and [distillation](http://dl.fbaipublicfiles.com/nat/distill_dataset.zip) datasets. Please build the binarized dataset on your own.
+
+
+## Train a model
+
+Then we can train a nonautoregressive model using the `translation_lev` task and a new criterion `nat_loss`.
+Use the `--noise` flag to specify the input noise used on the target sentences.
+In default, we run the task for *Levenshtein Transformer*, with `--noise='random_delete'`. Full scripts to run other models can also be found [here](./scripts.md).
+
+The following command will train a *Levenshtein Transformer* on the binarized dataset.
+
+```bash
+fairseq-train \
+    data-bin/wmt14_en_de_distill \
+    --save-dir checkpoints \
+    --ddp-backend=no_c10d \
+    --task translation_lev \
+    --criterion nat_loss \
+    --arch levenshtein_transformer \
+    --noise random_delete \
+    --share-all-embeddings \
+    --optimizer adam --adam-betas '(0.9,0.98)' \
+    --lr 0.0005 --lr-scheduler inverse_sqrt \
+    --min-lr '1e-09' --warmup-updates 10000 \
+    --warmup-init-lr '1e-07' --label-smoothing 0.1 \
+    --dropout 0.3 --weight-decay 0.01 \
+    --decoder-learned-pos \
+    --encoder-learned-pos \
+    --apply-bert-init \
+    --log-format 'simple' --log-interval 100 \
+    --fixed-validation-seed 7 \
+    --max-tokens 8000 \
+    --save-interval-updates 10000 \
+    --max-update 300000
+```
+
+## Translate
+
+Once a model is trained, we can generate translations using an `iterative_refinement_generator` which will based on the model's initial output and iteratively read and greedily refine the translation until (1) the model predicts the same translations for two consecutive iterations; or (2) the generator reaches the maximum iterations (`--iter-decode-max-iter`). Use `--print-step` to check the actual # of iteration for each sentence.
+
+For *Levenshtein Transformer*, it sometimes helps to apply a `--iter-decode-eos-penalty` (typically, 0~3) to penalize the model finishing generation too early and generating too short translations.
+
+
+For example, to generate with `--iter-decode-max-iter=9`:
+```bash
+fairseq-generate \
+    data-bin/wmt14_en_de_distill \
+    --gen-subset test \
+    --task translation_lev \
+    --path checkpoints/checkpoint_best.pt \
+    --iter-decode-max-iter 9 \
+    --iter-decode-eos-penalty 0 \
+    --beam 1 --remove-bpe \
+    --print-step \
+    --batch-size 400
+```
+In the end of the generation, we can see the tokenized BLEU score for the translation.
+
+
+## Citation
+
+```bibtex
+@article{gu2019levenshtein,
+  title={Levenshtein Transformer},
+  author={Gu, Jiatao and Wang, Changhan and Zhao, Jake},
+  journal={arXiv preprint arXiv:1905.11006},
+  year={2019}
+}
+```

examples/nonautoregressive_translation/scripts.md

@@ -0,0 +1,148 @@
+# Examples of Training scripts for Non-autoregressive Machine Translation models
+
+### Non-autoregressive Transformer (NAT, Gu et al., 2017)
+Note that we need to have an additional module to perform "length prediction" (`--length-loss-factor`) before generating the whole sequence.
+```bash
+fairseq-train \
+    data-bin/wmt14_en_de_distill \
+    --save-dir checkpoints \
+    --ddp-backend=no_c10d \
+    --task translation_lev \
+    --criterion nat_loss \
+    --arch nonautoregressive_transformer \
+    --noise full_mask \
+    --share-all-embeddings \
+    --optimizer adam --adam-betas '(0.9,0.98)' \
+    --lr 0.0005 --lr-scheduler inverse_sqrt \
+    --min-lr '1e-09' --warmup-updates 10000 \
+    --warmup-init-lr '1e-07' --label-smoothing 0.1 \
+    --dropout 0.3 --weight-decay 0.01 \
+    --decoder-learned-pos \
+    --encoder-learned-pos \
+    --pred-length-offset \
+    --length-loss-factor 0.1 \
+    --apply-bert-init \
+    --log-format 'simple' --log-interval 100 \
+    --fixed-validation-seed 7 \
+    --max-tokens 8000 \
+    --save-interval-updates 10000 \
+    --max-update 300000
+```
+
+### Non-autoregressive Transformer with Iterative Refinement (iNAT, Lee et al., 2018)
+Note that `--train-step` means how many iterations of refinement we used during training, and `--dae-ratio` controls the ratio of denoising auto-encoder training described in the original paper.
+```bash
+fairseq-train \
+    data-bin/wmt14_en_de_distill \
+    --save-dir checkpoints \
+    --ddp-backend=no_c10d \
+    --task translation_lev \
+    --criterion nat_loss \
+    --arch nonautoregressive_transformer \
+    --noise full_mask \
+    --share-all-embeddings \
+    --optimizer adam --adam-betas '(0.9,0.98)' \
+    --lr 0.0005 --lr-scheduler inverse_sqrt \
+    --min-lr '1e-09' --warmup-updates 10000 \
+    --warmup-init-lr '1e-07' --label-smoothing 0.1 \
+    --dropout 0.3 --weight-decay 0.01 \
+    --decoder-learned-pos \
+    --encoder-learned-pos \
+    --pred-length-offset \
+    --length-loss-factor 0.1 \
+    --train-step 4 \
+    --dae-ratio 0.5 \
+    --stochastic-approx \
+    --apply-bert-init \
+    --log-format 'simple' --log-interval 100 \
+    --fixed-validation-seed 7 \
+    --max-tokens 8000 \
+    --save-interval-updates 10000 \
+    --max-update 300000
+```
+
+### Insertion Transformer (InsT, Stern et al., 2019)
+Note that we need to specify the "slot-loss" (uniform or balanced tree) described in the original paper. Here we use `--label-tau` to control the temperature.
+
+```bash
+fairseq-train \
+    data-bin/wmt14_en_de_distill \
+    --save-dir checkpoints \
+    --ddp-backend=no_c10d \
+    --task translation_lev \
+    --criterion nat_loss \
+    --arch insertion_transformer \
+    --noise random_delete \
+    --share-all-embeddings \
+    --optimizer adam --adam-betas '(0.9,0.98)' \
+    --lr 0.0005 --lr-scheduler inverse_sqrt \
+    --min-lr '1e-09' --warmup-updates 10000 \
+    --warmup-init-lr '1e-07' --label-smoothing 0.1 \
+    --dropout 0.3 --weight-decay 0.01 \
+    --decoder-learned-pos \
+    --encoder-learned-pos \
+    --pred-length-offset \
+    --length-loss-factor 0.1 \
+    --apply-bert-init \
+    --log-format 'simple' --log-interval 100 \
+    --fixed-validation-seed 7 \
+    --max-tokens 8000 \
+    --save-interval-updates 10000 \
+    --max-update 300000
+```
+
+
+### Mask Predict (CMLM, Ghazvininejad et al., 2019)
+```bash
+fairseq-train \
+    data-bin/wmt14_en_de_distill \
+    --save-dir checkpoints \
+    --ddp-backend=no_c10d \
+    --task translation_lev \
+    --criterion nat_loss \
+    --arch cmlm_transformer \
+    --noise random_mask \
+    --share-all-embeddings \
+    --optimizer adam --adam-betas '(0.9,0.98)' \
+    --lr 0.0005 --lr-scheduler inverse_sqrt \
+    --min-lr '1e-09' --warmup-updates 10000 \
+    --warmup-init-lr '1e-07' --label-smoothing 0.1 \
+    --dropout 0.3 --weight-decay 0.01 \
+    --decoder-learned-pos \
+    --encoder-learned-pos \
+    --apply-bert-init \
+    --log-format 'simple' --log-interval 100 \
+    --fixed-validation-seed 7 \
+    --max-tokens 8000 \
+    --save-interval-updates 10000 \
+    --max-update 300000
+```
+
+
+
+
+### Levenshtein Transformer (LevT, Gu et al., 2019)
+```bash
+fairseq-train \
+    data-bin/wmt14_en_de_distill \
+    --save-dir checkpoints \
+    --ddp-backend=no_c10d \
+    --task translation_lev \
+    --criterion nat_loss \
+    --arch levenshtein_transformer \
+    --noise random_delete \
+    --share-all-embeddings \
+    --optimizer adam --adam-betas '(0.9,0.98)' \
+    --lr 0.0005 --lr-scheduler inverse_sqrt \
+    --min-lr '1e-09' --warmup-updates 10000 \
+    --warmup-init-lr '1e-07' --label-smoothing 0.1 \
+    --dropout 0.3 --weight-decay 0.01 \
+    --decoder-learned-pos \
+    --encoder-learned-pos \
+    --apply-bert-init \
+    --log-format 'simple' --log-interval 100 \
+    --fixed-validation-seed 7 \
+    --max-tokens 8000 \
+    --save-interval-updates 10000 \
+    --max-update 300000
+```