move benchmark utils into torch namespace (#41506)
Summary:
Move the timing utils to `torch.utils._benchmark`. I couldn't figure out how to get setuptools to pick it up and put it under `torch` unless it is in the `torch` directory. (And I think it has to be for `setup.py develop` anyway.)
I also modified the record function benchmark since `Timer` and `Compare` should always be available now.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/41506
Reviewed By: ngimel
Differential Revision: D22601460
Pulled By: robieta
fbshipit-source-id: 9cea7ff1dcb0bb6922c15b99dd64833d9631c37b
