code: def test_flatten(self):
      # Test that we flatten multiple Composite.
      x, y, z = floats("xyz")
      C = Composite([x, y], [x + y])
      CC = Composite([x, y], [C(x * y, y)])
      assert not isinstance(CC.outputs[0].owner.op, Composite)
  
      # Test with multiple outputs
      CC = Composite([x, y, z], [C(x * y, y), C(x * z, y)])
      # We don't flatten that case.
      assert isinstance(CC.outputs[0].owner.op, Composite)
  
imports: import numpy
  import pytest
  import pytensor
  import pytensor.tensor
  import tests.unittest_tools
  from pytensor.compile.mode import Mode
  from pytensor.graph.fg import FunctionGraph
  from pytensor.link.c.basic import DualLinker
  from pytensor.scalar.basic import ComplexError, Composite, InRange, ScalarType, add, and_, arccos, arccosh, arcsin, arcsinh, arctan, arctan2, arctanh, cast, complex64, constant, cos, cosh, deg2rad, eq, exp, exp2, expm1, float16, float32, floats, int8, int32, ints, invert, log, log1p, log2, log10, mean, mul, neq, rad2deg, reciprocal, sin, sinh, sqrt, switch, tan, tanh, true_div, uint8
  from pytensor.tensor.type import fscalar, imatrix, iscalar, matrix
  from tests.link.test_link import make_function
label_desc: Use image node_id 3 for calling the TestComposite obj's underlying member method code with example usage: obj.test_flatten() without return types