code: def test_builder_eval(self):
      v = mb.constexpr_cast(
          source_val=np.float16([1, 2]), output_dtype="fp32"
      )
      assert v.val is None
      np.testing.assert_allclose(
          np.float32([1, 2]), v.op.materialized_val_inference()
      )
  
imports: import itertools
  import numpy
  import pytest
  from coremltools.converters.mil import testing_reqs
  from coremltools.converters.mil.mil import Builder
  from coremltools.converters.mil.mil import types
  from coremltools.converters.mil.mil.ops.defs.iOS16 import constexpr_ops
  from coremltools.converters.mil.mil.ops.tests.iOS16 import backends
  from coremltools.converters.mil.mil.ops.tests.testing_utils import run_compare_builder
  from coremltools.converters.mil.testing_utils import get_op_types_in_program, ssa_fn
label_desc: Use image node_id 2 for calling the TestConstexprCast obj's underlying member method code with example usage: obj.test_builder_eval() without return types