fix convtransformer test
fix convtransformer test

Summary: in D23188945 (https://github.com/pytorch/fairseq/commit/bd1b35d9b7cb21b2e7c17201d831c17560265b67) looks like the sequence generator was modified to only support 2d src tokens, but for audio seems like we need 3d

Reviewed By: zhengwy888

Differential Revision: D23274482

fbshipit-source-id: 2e66c897c9d2c929158c2c9f858eac607af302bf

pytorch/fairseq,pytorch/fairseq,pytorch/fairseq