Use string based schema for exposing caffe2 ops (#19287)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19287
Since we now have a string-schema-based op registration API, we can also use it when exposing caffe2 operators.
Reviewed By: dzhulgakov
Differential Revision: D14931925
fbshipit-source-id: ec162469d2d94965e8c99d431c801ae7c43849c8
