Online backtranslation module

Co-authored-by: liezl200 <[email protected]>

0.4.0 -> 0.5.0

Changelog:
- 97b58b4: add Transformer model from Vaswani et al. (2017)
- b2374e5: faster Transformer inference with improved caching
- 2d27ae0: simulate large mini-batch training with delayed updates (`--update-freq`)
- 7ee1d28: add FP16 training support (`--fp16`)
- 2a84f46: faster inference by removing completed sentences from the batch
- 663fd80: batched interactive generation
- 4c2ef2d: add language modeling / gated convolutional model from Dauphin et al. (2017)
- b59815b: add Hierarchical Neural Story Generation model from Fan et al. (2018)
- ff68a9e: add FairseqTask to modularize task definitions (e.g., translation, language modeling)

Merge internal changes (facebookresearch#163)