Improve the quality of demucs model (#639)
Summary:
Original train batch size is 64, which doesn't work on T4. CPU training is too slow, so disable both GPU and CPU training.
Eval batch size 8 looks good:
Pull Request resolved: https://github.com/pytorch/benchmark/pull/639
Reviewed By: aaronenyeshi
Differential Revision: D33136298
Pulled By: xuzhao9
fbshipit-source-id: c7cc0172dd37a4e9dbc4025968f3b58aaccc0c24
