Fixes (#442)

Summary:
minor fixes:
1- adding fairseq logo
2- encoder padding for fconv self att
3- legacy ddp change
Pull Request resolved: https://github.com/pytorch/fairseq/pull/442

Differential Revision: D13651715

Pulled By: myleott

fbshipit-source-id: ac93c80f1dbffdfe03fbd4b8a8ea527aecb576a7