Add torch.no_grad() to inference tests (#1577)
Summary:
Adding the necessary `torch.no_grad()` contexts to inference tests.
Pull Request resolved: https://github.com/pytorch/benchmark/pull/1577
Reviewed By: aaronenyeshi
Differential Revision: D45274859
Pulled By: xuzhao9
fbshipit-source-id: c7ca00a63f4701cbd7ed2ae29debe84bcb22fdfc
