code: def export_flatten_with_default_axis() -> None:
      node = onnx.helper.make_node(
          "Flatten",
          inputs=["a"],
          outputs=["b"],  # Default value for axis: axis=1
      )
  
      shape = (5, 4, 3, 2)
      a = np.random.random_sample(shape).astype(np.float32)
      new_shape = (5, 24)
      b = np.reshape(a, new_shape)
      expect(
          node,
          inputs=[a],
          outputs=[b],
          name="test_flatten_default_axis",
      )
  
file_path: onnx/onnx/backend/test/case/node/flatten.py
imports: import numpy
  import onnx
  from onnx.backend.test.case.base import Base
  from onnx.backend.test.case.node import expect
label_desc: Use image node_id 2 for calling the Flatten obj's underlying member method code with example usage: obj.export_flatten_with_default_axis() without return types