OpenNMT-py v0.9.1

• New mechanism for MultiGPU training "1 batch producer / multi batch consumers"
  resulting in big memory saving when handling huge datasets
  thanks @pltrdy @francoishernandez

• New APEX AMP (mixed precision) API
  thanks @francoishernandez
  NB: you need to resintall Nvidia/Apex

• Option to overwrite shards when preprocessing

• Small fixes and add-ons