code: def testCannotInferNumFromUnknownShape(self):
      # Testing unknown shape in graph mode.
      with ops.Graph().as_default():
          x = array_ops.placeholder(np.float32)
          with self.assertRaisesRegex(
              ValueError,
              r"Cannot infer argument `num` from shape <unknown>",
          ):
              array_ops_stack.unstack(x)
  
imports: import numpy
  from tensorflow.python.framework import constant_op
  from tensorflow.python.framework import dtypes
  from tensorflow.python.framework import ops
  from tensorflow.python.framework import test_util
  from tensorflow.python.ops import array_ops
  from tensorflow.python.ops import array_ops_stack
  from tensorflow.python.ops import gradient_checker_v2
  from tensorflow.python.platform import test
label_desc: Use image node_id 8 for calling the UnstackOpTest obj's underlying member method code with example usage: obj.testCannotInferNumFromUnknownShape() without return types