seq2seq autoregressive flag check (#1618)
seq2seq autoregressive flag check (#1618)

Summary:
raise an exception if trying to use wav2vec seq2seq finetuning without autoregressive flag

Pull Request resolved: https://github.com/fairinternal/fairseq-py/pull/1618

Reviewed By: xuqiantong

Differential Revision: D26417249

Pulled By: alexeib

fbshipit-source-id: 777b6d170b0f8196746e03b399e4d7c21ac0b837

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq