Passing kwargs in setup_task in fairseq_task (#670)

Summary:
Pull Request resolved: https://github.com/pytorch/fairseq/pull/670

Pytorch-translate task needs to use extra arguments (such as vocabulary objects). By passing kwargs, we are able to have the ability to have extra arguments in setup_task

Reviewed By: akinh, pipibjc

Differential Revision: D15086810

fbshipit-source-id: 555f7976020eaac1febb8226f5a0055af0407ea6