class_docstr: Test RuntimePipeline in Pipeline
class_docstr_tok: ["Test","RuntimePipeline","in","Pipeline"]
code: def test_flat_map(self):
      p = pipe.input("x").flat_map("x", "y", p4).output("y")
      res = p(3).to_list()
      self.assertEqual(res, [[1], [2], [3]])
  
file_path: towhee/tests/unittests/runtime/test_dag_pipeline.py
imports: import unittest
  from towhee import pipe
label_desc: Use image node_id 6 for calling the TestDAGPipeline obj's underlying member method code with example usage: obj.test_flat_map() without return types