change f"{args}" to "{}".format(args) (#467)

Summary:
Although both are supported by Python 3.6, I think it would be better to unify the usage of string format function.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/467

Differential Revision: D13802506

Pulled By: myleott

fbshipit-source-id: 5c4877547b1c4ca806ab54c80ae483cfbaa7827a