Sockeye

Changed
- Multiple lexicons can now be specified with the `--restrict-lexicon` option:
- For a single lexicon: `--restrict-lexicon /path/to/lexicon`.
- For multiple lexicons: `--restrict-lexicon key1:/path/to/lexicon1 key2:/path/to/lexicon2 ...`.
- Use `--json-input` to specify the lexicon to use for each input, ex: `{"text": "some input string", "restrict_lexicon": "key1"}`.

Changed
- Updated to [MXNet 1.4.0](https://github.com/apache/incubator-mxnet/tree/1.4.0)
- Integration tests no longer check for equivalence of outputs with batch size 2

Fixed
- Made the length ratios per bucket change backwards compatible.

Changed
- Made sacrebleu a pip dependency and removed it from `sockeye_contrib`.

Added
- Data statistics at training time now compute mean and standard deviation of length ratios per bucket.
This information is stored in the model's config, but not used at the moment.

Added
- Added the `--fixed-param-strategy` option that allows fixing various model parameters during training via named strategies.
These include some of the simpler combinations from [Wuebker et al. (2018)](https://arxiv.org/abs/1811.01990) such as fixing everything except the first and last layers of the encoder and decoder (`all_except_outer_layers`). See the help message for a full list of strategies.