[RPC] Infer backend type if only options are given (#45065)
Summary:
Pull Request resolved: https://github.com/pytorch/pytorch/pull/45065
To preserve backwards compatibility with applications that were passing in some ProcessGroupRpcBackendOptions but were not explicitly setting backend=BackendType.PROCESS_GROUP, we're here now inferring the backend type from the options if only the latter ones are passed. If neither are passed, we'll default to TensorPipe, as before this change.
ghstack-source-id: 112586258
Test Plan: Added new unit tests.
Reviewed By: pritamdamania87
Differential Revision: D23814289
fbshipit-source-id: f4be7919e0817a4f539a50ab12216dc3178cb752
