Add code for mixture of experts (#521)

Summary:
Code for the paper: [Mixture Models for Diverse Machine Translation: Tricks of the Trade (Shen et al., 2019)](https://arxiv.org/abs/1902.07816).
Pull Request resolved: https://github.com/pytorch/fairseq/pull/521

Differential Revision: D14188021

Pulled By: myleott

fbshipit-source-id: ed5b1ed5ad9a582359bd5215fa2ea26dc76c673e