[pytorch] fix backend_type for backend/PG plugin (#93129)
Summary: For backend/PG plugin, use `ProcessGroup.BackendType.CUSTOM` to avoid uninitialized variable during `pg._register_backend` later
Test Plan: CI/CD and internal tests
Differential Revision: D42793222
Pull Request resolved: https://github.com/pytorch/pytorch/pull/93129
Approved by: https://github.com/H-Huang
