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

fairseq/criterions/fairseq_criterion.py

@@ -13,6 +13,7 @@ class FairseqCriterion(_Loss):
     def __init__(self, args, task):
         super().__init__()
         self.args = args
+        self.task = task
         self.padding_idx = task.target_dictionary.pad() if task.target_dictionary is not None else -100
 
     @staticmethod