Store task in the criterion base class
Store task in the criterion base class

Summary: Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/737

Differential Revision: D16377805

Pulled By: myleott

fbshipit-source-id: 1e090a02ff4fbba8695173f57d3cc5b88ae98bbf

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq