Sockeye

Added
- Optionally apply query, key and value projections to the source and target hidden vectors in the CNN model
before applying the attention mechanism. CLI parameter: `--cnn-project-qkv`.

Added
- A warning will be printed if the checkpoint decoder slows down training.

Added
- Exposing the xavier random number generator through `--weight-init-xavier-rand-type`.

Added
- Exposing MXNet's Nesterov Accelerated Gradient, Adadelta and Adadelta optimizers.

Added
- A tool that initializes embedding weights with pretrained word representations, `sockeye.init_embedding`.

Added
- Added support for Swish-1 (SiLU) activation to transformer models
([Ramachandran et al. 2017: Searching for Activation Functions](https://arxiv.org/pdf/1710.05941.pdf),
[Elfwing et al. 2017: Sigmoid-Weighted Linear Units for Neural Network Function Approximation
in Reinforcement Learning](https://arxiv.org/pdf/1702.03118.pdf)). Use `--transformer-activation-type swish1`.
- Added support for GELU activation to transformer models ([Hendrycks and Gimpel 2016: Bridging Nonlinearities and
Stochastic Regularizers with Gaussian Error Linear Units](https://arxiv.org/pdf/1606.08415.pdf).
Use `--transformer-activation-type gelu`.