Setup operator registration for distributed package (#31214)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/31214
This set up the basic infrastructure for distributed autograd and rpc to
bind their operators to TorchScript, since the whole distributed package
is builtin behind the `USE_DISTRIBUTED` flag, we separate the
registration and build it only when the flag is on.
Test Plan: Imported from OSS
Differential Revision: D19137160
fbshipit-source-id: ff47dc4c380ebe273fe0eea9e5e3fccfbd6466d7
