New operator registration MVP (#18161)

New operator registration MVP (#18161)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18161

This introduces version 0 for the new operator registration.

For now, it only works with kernels that are defined as stack-based functions.
This is actually not the intended public API for defining kernels, but it's the basis which is going to be used to define the public APIs (see diffs on top for them),
and it's also the API used for exposing caffe2 operators.

This diff also switches the mechanism for exposing caffe2 operators to the new mechanism.

Reviewed By: dzhulgakov

Differential Revision: D14514231

fbshipit-source-id: 454ab7b5b46a10203aa27b175400d23f818dd1df