message: Fix memory leak in nest's handling of CompositeTensors.
diff: @@ -550,7 +550,9 @@ ValueIteratorPtr GetValueIteratorForComposite(PyObject* nested) {
  if (PyErr_Occurred() || nested == nullptr) {
  return absl::make_unique<ErrorValueIterator>();
  }
  - return absl::make_unique<SingleValueIterator>(nested);
  + ValueIteratorPtr result = absl::make_unique<SingleValueIterator>(nested);
  + Py_DECREF(nested); // ValueIterator took ownership
  + return result;
  }
  return GetValueIterator(nested);
  }
  