Make error message for trying to train after make_generation_fast work correctly

Summary: https://github.com/pytorch/fairseq/blob/master/fairseq/trainer.py#L164 calls `train()` without any argument

Reviewed By: myleott

Differential Revision: D13599203

fbshipit-source-id: 3a096a6dd35a7a3f8309fbda3b54a36f606475e3