Add torch.backends.mkldnn.enabled flag (#25459)
Summary:
This PR is about add torch.backends.mkldnn.enabled flag said in https://github.com/pytorch/pytorch/issues/25186 which can be used disable mkldnn at runtime step as torch.backends.cudnn.enabled.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/25459
Differential Revision: D17258926
Pulled By: ezyang
fbshipit-source-id: e179ad364cc608fdaa7d0f37e2e762ceb5eda598
