[bugfix][distributed] improve p2p capability test (#5612)

[bugfix][distributed] do not error if two processes do not agree on p2p capability (#5612)

vllm/distributed/device_communicators/custom_all_reduce_utils.py

@@ -71,6 +71,7 @@ def consumer(batch_tgt: Sequence[int],
         if open_success:
             # modify the memory
             lib.cudaMemset(pointer, 2, 1024)
+            lib.cudaDeviceSynchronize()
             # use two queues to simulate barrier
             producer_queue.get()
             consumer_queue.put(0)
@@ -142,8 +143,13 @@ def can_actually_p2p(
     for src, tgt in zip(batch_src, batch_tgt):
         a = result_queue.get()
         b = result_queue.get()
-        assert a == b
-        result.append(a)
+        if a != b:
+            logger.warning(
+                "Two processes do not agree on the P2P access"
+                " status on %d -> %d, treat as disabled.", src, tgt)
+            result.append(False)
+        else:
+            result.append(a)
     return result