Expose c10 operators to caffe2 by operator name (#18160)

Expose c10 operators to caffe2 by operator name (#18160)

Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/18160

When exposing a c10 operator to the caffe2 frontend, don't use the operator schema but use the operator name instead.
This allows us to get rid of the existing mechanism for operator schema registration in a diff stacked on top.

Reviewed By: dzhulgakov

Differential Revision: D14513420

fbshipit-source-id: 6b08a9c6d9497eaf18b62361dd44bc07c7b4b76b