Don't make factory methods create a tensor and then immediately copy it (#17565)
Summary:
Create a `make_variable` override that moves out of a tensor instead of going through `shallow_copy_and_detach`. Call this override from factory methods like `empty` that create a brand new tensor, do nothing with it, and then copy it into a variable.
Will update this with actual numbers, but it seems to get rid of around 20-40% of the overhead of calling `torch.empty(0)`
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17565
Differential Revision: D14266130
Pulled By: umanwizard
fbshipit-source-id: f57d5f2ca3f80ee8ee96d50f905e852fd10db941
