Sockeye

Changed
- Update to MXNet 1.3.1. Removed requirements/requirements.gpu-cu{75,91}.txt as CUDA 7.5 and 9.1 are deprecated.

Fixed
- Performance optimization to skip the softmax operation for single model greedy decoding is now only applied if no translation scores are required in the output.

Added
- Full training state is now returned from EarlyStoppingTrainer's fit().
Changed
- Training state cleanup will not be performed for training runs that did not converge yet.
- Switched to portalocker for locking files (Windows compatibility).

Added
- Added nbest translation, exposed as `--nbest-size`. Nbest translation means to not only output the most probable translation according to a model, but the top n most probable hypotheses. If `--nbest-size > 1` and the option `--output-type` is not explicitly specified, the output type will be changed to one JSON list of nbest translations per line. `--nbest-size` can never be larger than `--beam-size`.

Changed
- Changed `sockeye.rerank` CLI to be compatible with nbest translation JSON output format.

Added
- Added `sockeye.score` CLI for quickly scoring existing translations ([documentation](tutorials/scoring.md)).
Fixed
- Entry-point clean-up after the contrib/ rename

Changed
- Update to MXNet 1.3.0.post0