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