bug fix: use cls.load_dictionary for multilingual translation
bug fix: use cls.load_dictionary for multilingual translation

Summary:
Currently, multilingual translation imports Dictionary and calls its load function.

However, this does not permit extending the class with a different load_dictionary function to modify its behavior.

Reviewed By: myleott, chtran

Differential Revision: D22441356

fbshipit-source-id: b0ef159182b15adb479b117581ddcd2f65724980

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq