[BUG] Fix async rlhf tests (#35811)

Signed-off-by: ahao-anyscale <ahao@anyscale.com>

.buildkite/test_areas/distributed.yaml

@@ -149,7 +149,7 @@ steps:
   num_devices: 2
   commands:
     - pytest -v -s tests/distributed/test_context_parallel.py
-    # - VLLM_ALLOW_INSECURE_SERIALIZATION=1 python3 examples/offline_inference/new_weight_syncing/rlhf_async_new_apis.py --- failing, need to re-enable
+    - VLLM_ALLOW_INSECURE_SERIALIZATION=1 python3 examples/offline_inference/new_weight_syncing/rlhf_async_new_apis.py
     - VLLM_USE_DEEP_GEMM=1 VLLM_LOGGING_LEVEL=DEBUG python3 examples/offline_inference/data_parallel.py --model=Qwen/Qwen1.5-MoE-A2.7B -tp=1 -dp=2 --max-model-len=2048 --all2all-backend=deepep_high_throughput
     - pytest -v -s tests/v1/distributed/test_dbo.py
 

vllm/v1/worker/gpu_worker.py

@@ -1006,6 +1006,10 @@ class Worker(WorkerBase):
                 load_weights=load_weights_direct,
             )
 
+        # NCCL broadcast/packed path are asynchronous.
+        # Sync here so the next step uses the new weights.
+        torch.accelerator.synchronize()
+
     def shutdown(self) -> None:
         # has_kv_transfer_group can be None during interpreter shutdown.
         if ensure_kv_transfer_shutdown is not None: