Simplify fairseq multihead attention (#888)
Simplify fairseq multihead attention (#888)

Summary:
Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/888

We want to simplify multihead attention and get rid of the dynamic in_proj_weight logic. Sending the diff early for feedback, will have further changes as I try to fix breaking tests

Reviewed By: edunov

Differential Revision: D17912661

fbshipit-source-id: 0e6319fc694d8ec5187d1c2fefe5839d9d522186

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq