Support third-party devices to use the init_process_group method with… (#107113)
…out specifying the Backend
When init_process_group is not been done before, it will automatically apply init_process_group within Devicemesh without specifying the backend. Thus, when a third-party device want to use Devicemesh without doing init_process_group before, there comes a problem. In this PR, add a default_device_backend_map for third-party device users to add their backends to this map when they register their backends to pytorch firstly. When doing init_process_group without parameter backend, it will init the backends in this map. Thus, a third-party user can use init_process_group method without specifying the Backend.
Pull Request resolved: https://github.com/pytorch/pytorch/pull/107113
Approved by: https://github.com/wanchaol
