message: No segfault in GradientTape with partially unknown shapes.
diff: @@ -1403,9 +1403,13 @@ class PyVSpace
  PyObject* arglist =
  Py_BuildValue("(O)", reinterpret_cast<PyObject*>(tensor));
  PyObject* result = PyEval_CallObject(num_elements_, arglist);
  + Py_DECREF(arglist);
  + if (result == nullptr) {
  + // The caller detects whether a python exception has been raised.
  + return -1;
  + }
  tensorflow::int64 r = MakeInt(result);
  Py_DECREF(result);
  - Py_DECREF(arglist);
  return r;
  }
  