class_docstr: PipelineManager Test
class_docstr_tok: ["PipelineManager","Test"]
code: def test_preload(self):
      towhee_dag_test = copy.deepcopy(self.dag_dict)
      runtime_pipeline = RuntimePipeline(towhee_dag_test)
      runtime_pipeline.preload()
      self.assertEqual(len(runtime_pipeline._operator_pool._all_ops), 4)
  
imports: import copy
  import weakref
  import unittest
  from towhee import pipe
  from towhee.operator import PyOperator
  from towhee.tools.data_visualizer import DataVisualizer
  from towhee.tools.profilers import PerformanceProfiler
  from towhee.runtime.runtime_pipeline import RuntimePipeline
  from towhee.runtime.operator_manager import OperatorRegistry
label_desc: Use image node_id 3 for calling the TestPipelineManager obj's underlying member method code with example usage: obj.test_preload() without return types