code: def test_walk_bfs_post_order_fails():
      relu = Relu(5)
      with pytest.raises(ValueError, match="Invalid order"):
          relu.walk(order="dfs_post_order")
  
imports: import threading
  import time
  from collections import Counter
  import numpy
  import pytest
  from thinc.api import Adam, CupyOps, Dropout, Linear, Model, Relu, Shim, Softmax, chain, change_attr_values, concatenate, set_dropout_rate, use_ops, with_debug, wrap_model_recursive
  from thinc.compat import has_cupy_gpu
  from ..util import make_tempdir
label_desc: Use image node_id 33 for calling a global function with example usage: test_walk_bfs_post_order_fails() without return types