Fix LegacyDistributedDataParallel (#2447)
Fix LegacyDistributedDataParallel (#2447)

Summary:
The "need_reduction" check was to help any legacy callers who might not be using the fairseq Trainer.

Unfortunately the mechanism doesn't work during validation, since there we call forward multiple times without calling backward.

Let's remove the check -- pretty much everyone should be using fairseq Trainer.

Pull Request resolved: https://github.com/pytorch/fairseq/pull/2447

Reviewed By: cndn, msbaines

Differential Revision: D23005086

Pulled By: myleott

fbshipit-source-id: 3a58342024096f2f3abc7bf1cdcce75e56a06aa0

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq