Reset both WPS and UPS on first minibatch (#891)
Reset both WPS and UPS on first minibatch (#891)

Summary:
Makes more sense to reset either both meters or neither of them.
Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/891

Differential Revision: D18109027

Pulled By: jma127

fbshipit-source-id: f63baed9a6b928a6f591a76e69ef6e9c524e4398

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq