Replace string literals for special symbols in Torchscript decoder
Replace string literals for special symbols in Torchscript decoder

Summary: repeat title. Due to TorchScript constraints it's not trivial to sync fairseq dictionary symbols in VocabConstants.

Reviewed By: theweiho

Differential Revision: D23774348

fbshipit-source-id: ec8db5ca41ee448ed317e76eabd4971f833015cc

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq