Add if ops support for onnxifi and ssa-rewrite (#19585)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/19585
Originally we will unroll all If op to many different subnets;
Now we will not unroll it anymore, but just add all external input of its subnet to the If op, and ssa-rewrite all external input/outputs. That would be enough.
Reviewed By: yinghai
Differential Revision: D15038139
fbshipit-source-id: 8532216d8749068acd5558ad0d8cb1d98463a063
