[c10d] Split custom class bindings out of python binding code (#58992)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/58992
Currently, we define Torchbind custom classes in the same place that we define Python bindings.
This is nice from a code location perspective, but has two downsides:
1. These custom classes are not available in a C++-only build.
2. These break when included in torch::deploy.
Some explanation on the second issue: torch::deploy creates many Python
interpreters, and creates a full copy of all the bindings for each one. This
will run the static initialization code once for each copy of the bindings,
leading to multiple registration of the custom classes (and therefore an
error).
This PR splits out the relevant custom class binding code into its own source
file to be included in libc10d, which can be compiled and statically
initialized a single time and linked against from the c10d python bindings.
ghstack-source-id: 130168942
Test Plan: CI
Reviewed By: wconstab
Differential Revision: D28690832
fbshipit-source-id: 3c5e3fff28abb8bcdb4a952794c07de1ee2ae5a8
