class_docstr: Unit test for Config.
code: def test_concat(self):
      pipe0 = towhee.pipe.input("a")
      pipe1 = pipe0.map("a", "b", lambda x: x + 1)
      pipe2 = pipe0.map("a", "c", lambda x: x + 2)
      pipe = pipe2.concat(pipe1).output("a", "b", "c")
      graph = pipe.preload()
      self.assertEqual(
          [v.name for _, v in graph._nodes.items()],
          ["_input", "lambda-0", "lambda-1", "concat-2", "_output"],
      )
  
imports: import unittest
  import towhee
  from towhee.runtime import ops, register
label_desc: Use image node_id 9 for calling the TestConfig obj's underlying member method code with example usage: obj.test_concat() without return types