Fix loading of very old checkpoints (#1512)
Fix loading of very old checkpoints (#1512)

Summary:
Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/1512

See https://github.com/pytorch/fairseq/issues/3032 for context

Test Plan: Imported from OSS

Reviewed By: alexeib

Differential Revision: D25570470

Pulled By: myleott

fbshipit-source-id: 9227b1ca36cd81ff72acdb5e03fd574e3e8769be

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq