chore: import upstream snapshot with attribution

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+# Neural Machine Translation with Byte-Level Subwords
+
+https://arxiv.org/abs/1909.03341
+
+We provide an implementation of byte-level byte-pair encoding (BBPE), taking IWSLT 2017 Fr-En translation as
+example.
+
+## Data
+Get data and generate fairseq binary dataset:
+```bash
+bash ./get_data.sh
+```
+
+## Model Training
+Train Transformer model with Bi-GRU embedding contextualization (implemented in `gru_transformer.py`):
+```bash
+# VOCAB=bytes
+# VOCAB=chars
+VOCAB=bbpe2048
+# VOCAB=bpe2048
+# VOCAB=bbpe4096
+# VOCAB=bpe4096
+# VOCAB=bpe16384
+```
+```bash
+fairseq-train "data/bin_${VOCAB}" --task translation --user-dir examples/byte_level_bpe/gru_transformer \
+    --arch gru_transformer --encoder-layers 2 --decoder-layers 2 --dropout 0.3 --share-all-embeddings \
+    --optimizer adam --adam-betas '(0.9, 0.98)' \
+    --lr 5e-4 --lr-scheduler inverse_sqrt --warmup-updates 4000 \
+    --criterion label_smoothed_cross_entropy --label-smoothing 0.1 \
+    --log-format 'simple' --log-interval 100 --save-dir "checkpoints/${VOCAB}" \
+    --batch-size 100 --max-update 100000 --update-freq 2
+```
+
+## Generation
+`fairseq-generate` requires bytes (BBPE) decoder to convert byte-level representation back to characters:
+```bash
+# BPE=--bpe bytes
+# BPE=--bpe characters
+BPE=--bpe byte_bpe --sentencepiece-model-path data/spm_bbpe2048.model
+# BPE=--bpe sentencepiece --sentencepiece-model data/spm_bpe2048.model
+# BPE=--bpe byte_bpe --sentencepiece-model-path data/spm_bbpe4096.model
+# BPE=--bpe sentencepiece --sentencepiece-model data/spm_bpe4096.model
+# BPE=--bpe sentencepiece --sentencepiece-model data/spm_bpe16384.model
+```
+
+```bash
+fairseq-generate "data/bin_${VOCAB}" --task translation --user-dir examples/byte_level_bpe/gru_transformer \
+    --source-lang fr --gen-subset test --sacrebleu --path "checkpoints/${VOCAB}/checkpoint_last.pt" \
+    --tokenizer moses --moses-target-lang en ${BPE}
+```
+When using `fairseq-interactive`, bytes (BBPE) encoder/decoder is required to tokenize input data and detokenize model predictions:
+```bash
+fairseq-interactive "data/bin_${VOCAB}" --task translation --user-dir examples/byte_level_bpe/gru_transformer \
+    --path "checkpoints/${VOCAB}/checkpoint_last.pt" --input data/test.fr --tokenizer moses --moses-source-lang fr \
+    --moses-target-lang en ${BPE} --buffer-size 1000 --max-tokens 10000
+```
+
+## Results
+| Vocabulary    | Model  | BLEU |
+|:-------------:|:-------------:|:-------------:|
+| Joint BPE 16k ([Kudo, 2018](https://arxiv.org/abs/1804.10959)) | 512d LSTM 2+2 | 33.81 |
+| Joint BPE 16k | Transformer base 2+2 (w/ GRU) | 36.64 (36.72) |
+| Joint BPE 4k | Transformer base 2+2 (w/ GRU) | 35.49 (36.10) |
+| Joint BBPE 4k | Transformer base 2+2 (w/ GRU) | 35.61 (35.82) |
+| Joint BPE 2k | Transformer base 2+2 (w/ GRU) | 34.87 (36.13) |
+| Joint BBPE 2k | Transformer base 2+2 (w/ GRU) | 34.98 (35.43) |
+| Characters | Transformer base 2+2 (w/ GRU) | 31.78 (33.30) |
+| Bytes | Transformer base 2+2 (w/ GRU) | 31.57 (33.62) |
+
+
+## Citation
+```
+@misc{wang2019neural,
+    title={Neural Machine Translation with Byte-Level Subwords},
+    author={Changhan Wang and Kyunghyun Cho and Jiatao Gu},
+    year={2019},
+    eprint={1909.03341},
+    archivePrefix={arXiv},
+    primaryClass={cs.CL}
+}
+```
+
+
+## Contact
+Changhan Wang ([changhan@fb.com](mailto:changhan@fb.com)),
+Kyunghyun Cho ([kyunghyuncho@fb.com](mailto:kyunghyuncho@fb.com)),
+Jiatao Gu ([jgu@fb.com](mailto:jgu@fb.com))