Add args and sys.argv to tensorboard (#673)

Summary:
Log fairseq's `args` and `sys.argv` in tensorboard to easily identify run hyperparameters from within tensorboard.

The idea was suggested in https://twitter.com/Thom_Wolf/status/1106300583835766786
Pull Request resolved: https://github.com/pytorch/fairseq/pull/673

Differential Revision: D15114159

Pulled By: myleott

fbshipit-source-id: d48133a7f629dffe984836712390c317916cf413