GitLab

Also includes following changes for inter-layer model-parallel implementation:
- Refactoring of model implementations
- Training loop changes to support inter-layer communication using `ring_exchange`
- New groups for inter-layer communication
- Checkpoint changes
- Command line arguments

preprocess_data:
- Adds ability to not split sentences. This is used for gpt2 datasets.

- Adds ability to create multiple datasets from different json keys,
this is current untested.

indexed_dataset:
- Add new "get" function to get a portion of an entry.

Some cleanup of types, etc. in the helpers cpp code.

Also removed the fix_lua_indexing stuff from fairseq.