GitLab

Summary:
NAT productionization diff

(1) Integrate NAT model training / Evaluation in LATTE base training workflow.
(2) Make NAT tracing compliant. Since it calls into Fairseq transformer, we need to refactor the code and I created a ~copy of it named fb_tracing_transformer.
(3) Decoder side C++ code is landed in the diff earlier.

Reviewed By: xianxl

Differential Revision: D17888324

fbshipit-source-id: ef4ef195fddd360da921502adcef82b087e46ce6

Summary:
Add ensemble wrappers to the levenshtein NAT.
Levenshtein
Final softmax ensemble over the pipeline of three steps: deletion, placeholder insertion, and word selection.
1. Deletion
2. Placeholder Insertion
3. Word Selection

Each step involves scoring, averaging the scores over the ensemble, and then make hard decisions with argmax. Then next step follows. We cannot do the three steps in parallel by design.

Reviewed By: kahne

Differential Revision: D17723202

fbshipit-source-id: 05f7a4fcd922a972cc4796ca397e8220f0b4d53e

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