[Bugfix] Do not block diffusion client shutdown on a dead subprocess (#7812)

Signed-off-by: cr-gao <gaochenrui@sjtu.edu.cn>

tests/diffusion/test_multiproc_engine_concurrency.py

@@ -1431,7 +1431,7 @@ class TestStageDiffusionClientErrorPropagation:
         )
 
         assert result is None
-        client._request_socket.send.assert_called_once_with(b"encoded-rpc")
+        client._request_socket.send.assert_called_once_with(b"encoded-rpc", flags=zmq.NOBLOCK)
         assert rpc_id not in client._pending_rpcs