Add LegacyDistributedDataParallel in place of no_c10d (#370)

Summary:
This should bring back the speedup with --update-freq that we reported in the Scaling Neural Machine Translation paper.
Pull Request resolved: https://github.com/pytorch/fairseq/pull/370

Differential Revision: D13100281

Pulled By: myleott

fbshipit-source-id: 4a81b51bb7390a197add314a4be5512bbf68c085