add separate path for op with JIT (#21210)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/21210
This diff introduces a new path to run op with JIT. There are two steps involved here:
1. Users need to script the op. This should happen in the `init` method.
2. The generated graph from step1 is passed to `jit_forward` which will be executed by the benchmark backend
Reviewed By: zheng-xq
Differential Revision: D15460831
fbshipit-source-id: 48441d9cd4be5d0acebab901f45544616e6ed2ee
