add max_keep_size (#2124)
add max_keep_size (#2124)

Summary:
Set max_keep_size to filter long utterances. Needed when trained on labeled datasets with long utterances.

Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/2124

Reviewed By: Abdel-rahmanMohamed

Differential Revision: D30046509

Pulled By: wnhsu

fbshipit-source-id: ec52ae0997284a05295dff35626927a71c78cf52

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq