[BE] delete frontend.cpp (#67400)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/67400
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: 143910066
ghstack-source-id: 143910066
Test Plan: wait for ci
Reviewed By: navahgar
Differential Revision: D31979270
fbshipit-source-id: 6ceb8b53d67ab8f9aef44b34da79346dfbb51225
