[Bugfix] Add synchronize to prevent possible data race (vllm-project#…

…6788)

Co-authored-by: Lucas Wilkinson <[email protected]>

vllm/distributed/parallel_state.py

@@ -243,6 +243,13 @@ def graph_capture(
         ca_comm = self.ca_comm
         maybe_ca_context = nullcontext(
         ) if ca_comm is None else ca_comm.capture()
+
+        # ensure all initialization operations complete before attempting to
+        # capture the graph on another stream
+        curr_stream = torch.cuda.current_stream()
+        if curr_stream != stream:
+            stream.wait_stream(curr_stream)
+
         with torch.cuda.stream(stream), maybe_ca_context:
             # In graph mode, we have to be very careful about the collective
             # operations. The current status is: