Move task import in MultilingualTransformer to fix circular dependencies

Summary: Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/687

Differential Revision: D16005399

Pulled By: myleott

fbshipit-source-id: bf099c17e2095394acc452e9abcb4ee04afd0426

fairseq/models/multilingual_transformer.py

@@ -20,7 +20,6 @@ from fairseq.models.transformer import (
     TransformerEncoder,
     TransformerDecoder,
 )
-from fairseq.tasks.multilingual_translation import MultilingualTranslationTask
 
 
 @register_model('multilingual_transformer')
@@ -59,6 +58,7 @@ class MultilingualTransformerModel(FairseqMultiModel):
     @classmethod
     def build_model(cls, args, task):
         """Build a new model instance."""
+        from fairseq.tasks.multilingual_translation import MultilingualTranslationTask
         assert isinstance(task, MultilingualTranslationTask)
 
         # make sure all arguments are present in older models