Add nbytes, itemsize, element_size to at::Tensor. (#17810)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/17810
Partially addresses #12728. Also, switch the element_size bindings
to use the new function, rather than the method on Type.
We don't add Python bindings yet, as they need to be special
(they will be properties.)
Differential Revision: D14388790
fbshipit-source-id: 294183d0c8a59b0c13f2bf21d6f1cd557333e83b
