[Core][Test] fix function name typo in custom allreduce (#4750)

tests/distributed/test_custom_all_reduce.py

@@ -25,7 +25,7 @@ def graph_allreduce(world_size, rank, distributed_init_port):
     init_test_distributed_environment(1, world_size, rank,
                                       distributed_init_port)
 
-    custom_all_reduce.init_custom_all_reduce()
+    custom_all_reduce.init_custom_ar()
     for sz in test_sizes:
         for dtype in [torch.float32, torch.float16, torch.bfloat16]:
             with custom_all_reduce.capture():
@@ -61,7 +61,7 @@ def eager_allreduce(world_size, rank, distributed_init_port):
                                       distributed_init_port)
 
     sz = 1024
-    custom_all_reduce.init_custom_all_reduce()
+    custom_all_reduce.init_custom_ar()
     fa = custom_all_reduce.get_handle()
     inp = torch.ones(sz, dtype=torch.float32, device=device)
     out = fa.all_reduce_unreg(inp)

vllm/distributed/device_communicators/custom_all_reduce.py

@@ -52,6 +52,10 @@ def init_custom_ar() -> None:
             "current process. This might be the case if 'CUDA_VISIBLE_DEVICES'"
             " is set.")
         return
+
+    # we only use a subset of GPUs here
+    # so we only need to check the nvlink connectivity of these GPUs
+    num_dev = world_size
     # test nvlink first, this will filter out most of the cases
     # where custom allreduce is not supported
     cuda_visible_devices = envs.CUDA_VISIBLE_DEVICES