op {   name: "DatasetToSingleElement"   input_arg {     name: "dataset"     type: DT_VARIANT   }   output_arg {     name: "components"     type_list_attr: "output_types"   }   attr {     name: "output_types"     type: "list(type)"     has_minimum: true     minimum: 0   }   attr {     name: "output_shapes"     type: "list(shape)"     has_minimum: true     minimum: 0   } 