Summary:
Notable (possibly breaking) changes:
- d45db80: Remove checkpoint utility functions from utils.py into checkpoint_utils.py
- f2563c2: Move LM definitions into separate files
- dffb167: Updates to model API:
  - `FairseqModel` -> `FairseqEncoderDecoderModel`
  - add `FairseqDecoder.extract_features` and `FairseqDecoder.output_layer`
  - `encoder_out_dict` -> `encoder_out`
  - rm unused `remove_head` functions
- 34726d5: Move `distributed_init` into `DistributedFairseqModel`
- cf17068: Simplify distributed launch by automatically launching multiprocessing on each node for all visible GPUs (allows launching just one job per node instead of one per GPU)
- d45db80: Change default LR scheduler from `reduce_lr_on_plateau` to `fixed`
- 96ac28d: Rename `--sampling-temperature` -> `--temperature`
- fc1a19a: Deprecate dummy batches
- a1c997b: Add memory mapped datasets
- 0add50c: Allow cycling over multiple datasets, where each one becomes an "epoch"

Plus many additional features and bugfixes
Pull Request resolved: https://github.com/pytorch/fairseq/pull/817

Differential Revision: D15913844

Pulled By: myleott

fbshipit-source-id: d5b5d678efdd9dd3e4d7ca848ddcf1ec2b21bf6b