Raises TypeError if `node_def` does not match the expectations.

def _check_asset_node_def(node_def):
  """Raises TypeError if `node_def` does not match the expectations."""
  if node_def.op != "Const":
    raise TypeError("Asset node must be of type constant.")
  if tf.as_dtype(node_def.attr["dtype"].type) != tf.string:
    raise TypeError("Asset node must be of dtype string.")
  if len(node_def.attr["value"].tensor.string_val) != 1:
    raise TypeError("Asset node must be a scalar.")