[BE] delete frontend.cpp (#66581)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/66581
c10d/frontend.cpp was originally proposed to introduce pure C++ API and use TorcBind to share python level API with TorchScript. This is no longer needed, so delete this to reduce code redundancy.
ghstack-source-id: 141336190
Test Plan: wait for ci
Reviewed By: rohan-varma
Differential Revision: D31627107
fbshipit-source-id: 07d30d280c25502a222a74c2c65dfa4069ed8713
