[BugFix] Fix torch distributed stateless PG backend init (#14870)

Signed-off-by: Nick Hill <nhill@redhat.com>

vllm/distributed/utils.py

@@ -299,13 +299,10 @@ def stateless_init_torch_distributed_process_group(
     # different systems (e.g. RPC) in case the store is multi-tenant.
     prefix_store = PrefixStore(init_method, store)
 
-    pg_options = ProcessGroup.Options(backend=backend, timeout=timeout)
-
     pg: ProcessGroup = ProcessGroup(
         prefix_store,
         group_rank,
         group_size,
-        pg_options,
     )
 
     if backend == "gloo":
@@ -327,7 +324,10 @@ def stateless_init_torch_distributed_process_group(
                                          backend_options)
         backend_type = ProcessGroup.BackendType.NCCL
         device = torch.device("cuda")
+    else:
+        raise RuntimeError(f"Unsupported torch distributed backend: {backend}")
 
+    pg._set_default_backend(backend_type)
     backend_class._set_sequence_number_for_group()
 
     pg._register_backend(device, backend_type, backend_class)