Sockeye

Added

- Added an optional ability to cache encoder outputs of model.

Fixed

- Fixed a bug where the training state object was saved to disk before training metrics were added to it, leading to an inconsistency between the training state object and the metrics file (see 859).

Fixed

- When loading a shard in Horovod mode, there is now a check that each non-empty bucket contains enough sentences to cover each worker's slice. If not, the bucket's sentences are replicated to guarantee coverage.

Fixed

- Fixed a bug where sampling translation fails because an array is created in the wrong context.

Added

- Added `layers.SSRU`, which implements a Simpler Simple Recurrent Unit as described in
Kim et al, "From Research to Production and Back: Ludicrously Fast Neural Machine Translation" WNGT 2019.

- Added `ssru_transformer` option to `--decoder`, which enables the usage of SSRUs as a replacement for the decoder-side self-attention layers.

Changed

- Reduced the number of arguments for `MultiHeadSelfAttention.hybrid_forward()`.
`previous_keys` and `previous_values` should now be input together as `previous_states`, a list containing two symbols.

Fixed

- Fixed batch sizing error introduced in version 2.1.12 (c00da52) that caused batch sizes to be multiplied by the number of devices. Batch sizing now works as documented (same as pre-2.1.12 versions).
- Fixed `max-word` batching to properly size batches to a multiple of both `--batch-sentences-multiple-of` and the number of devices.