fix varlen ci test hang

Author: liangel-02

torchtitan/experiments/rl/tests/test_bitwise_parity.py

@@ -679,13 +679,19 @@ def setUpClass(cls):
 
     @classmethod
     def tearDownClass(cls):
+        if dist.is_initialized():
+            dist.barrier()
         if hasattr(cls, "engine"):
-            cls.engine.engine_core.shutdown()
+            renderer = getattr(cls.engine, "renderer", None)
+            if renderer is not None:
+                renderer.shutdown()
             del cls.engine
         if hasattr(cls, "model"):
             del cls.model
         gc.collect()
         torch.cuda.empty_cache()
+        if dist.is_initialized():
+            dist.destroy_process_group()
 
     def _assert_logprobs_equal(self, name, a, b, label_a="A", label_b="B"):
         """Assert two logprob sequences are bitwise identical."""