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Changes:
- 7d19e36: Add `--sampling` flag to generate.py to sample instead of doing beam search
- c777340: Add `scripts/average_checkpoints.py` to average multiple checkpoints into a combined model
- 3ea882c: Add `--max-update` option to train.py to stop training after a given number of updates
- small bugfixes for distributed training, LSTM, inverse square root LR scheduler

This PR includes breaking API changes to modularize fairseq-py and adds support for distributed training across multiple nodes.

Changes:
- c7033ef: add support for distributed training! See updated README for usage.
- e016299: modularize fairseq-py, adding support for register_model, register_criterion, register_optimizer, etc.
- 154e440: update LSTM implementation to use PackedSequence objects in the encoder, better following best practices and improving perf
- 90c2973 and 1da6265: improve unit test coverage

* Split generate.py to generate.py and interactive.py and refactor code

The main motivation behind these changes is to try to decorrelate use
cases in order to implement future improvements such as unk replacement
with original string during evaluation on test and writing predictions
to output file.
The previous implementation worked well but I found it difficult to
integrate these future improvements.

* Add --replace-unk arg to be used without align dict

Replacing <unk> tokens can be beneficial even without an alignment
dictionary.