class_docstr: Test Pipeline
class_docstr_tok: ["Test","Pipeline"]
code: def test_concat_raise(self):
      p1 = Pipeline.input("a").map("a", "b", lambda x: x + 1)
      with self.assertRaises(ValueError):
          p1.concat().concat().output("b")
      with self.assertRaises(ValueError):
          p1.concat("a").output("b")
  
imports: import unittest
  from towhee.operator.nop import NOPNodeOperator
  from towhee.operator import PyOperator
  from towhee.runtime.pipeline import Pipeline
  from towhee.runtime.data_queue import Empty
  from towhee.runtime import ops, register
  from towhee.runtime.runtime_pipeline import RuntimePipeline
label_desc: Use image node_id 21 for calling the TestPipeline obj's underlying member method code with example usage: obj.test_concat_raise() without return types