message: fix string representation of scalar types
diff: @@ -119,17 +119,12 @@ std::string DataTypeUtils::ToString(
  const std::string& right) {
  switch (type_proto.value_case()) {
  case TypeProto::ValueCase::kTensorType: {
  - if (type_proto.tensor_type().has_shape() &&
  - type_proto.tensor_type().shape().dim_size() == 0) {
  - // Scalar case.
  - return left + ToDataTypeString(type_proto.tensor_type().elem_type()) +
  - right;
  - } else {
  + // Note: We do not distinguish tensors with zero rank (a shape consisting of
  + // an empty sequence of dimensions) here.
  return left + "tensor(" +
  ToDataTypeString(type_proto.tensor_type().elem_type()) + ")" +
  right;
  }
  - }
  #ifdef ONNX_ML
  case TypeProto::ValueCase::kSequenceType: {
  return ToString(type_proto.sequence_type().elem_type(), left + "seq(", ")" + right);
  