Sockeye

Added
- Added support for the use of adding target prefix and target prefix factors to the input in JSON format during inference.

Added
- Added support for the use of adding source prefixes to the input in JSON format during inference.

Changed
- Optimized creation of source length mask by using `expand` instead of `repeat_interleave`.

Changed
- Updated torch dependency to 1.10.x (`torch>=1.10.0,<1.11.0`)

Sockeye is now exclusively based on Pytorch.

Changed
- Renamed `x_pt` modules to `x`. Updated entry points in `setup.py`.

Removed
- Removed MXNet from the codebase
- Removed device locking / GPU acquisition logic. Removed dependency on `portalocker`.
- Removed arguments `--softmax-temperature`, `--weight-init-*`, `--mc-dropout`, `--horovod`, `--device-ids
- Removed all MXNet-related tests

Fixed
- Fixed GPU-based scoring by copying to cpu tensor first before converting to numpy.