code: def levels_na():
      cov_type = h2o.import_file(
          pyunit_utils.locate("bigdata/laptop/covtype/covtype.full.csv")
      )
      assert cov_type.levels() == [
          [],
          [],
          [],
          [],
          [],
          [],
          [],
          [],
          [],
          [],
          ["area_0", "area_1", "area_2", "area_3"],
          [
              "type_0",
              "type_1",
              "type_10",
              "type_11",
              "type_12",
              "type_13",
              "type_14",
              "type_15",
              "type_16",
              "type_17",
              "type_18",
              "type_19",
              "type_2",
              "type_20",
              "type_21",
              "type_22",
              "type_23",
              "type_24",
              "type_25",
              "type_26",
              "type_27",
              "type_28",
              "type_29",
              "type_3",
              "type_30",
              "type_31",
              "type_32",
              "type_33",
              "type_34",
              "type_35",
              "type_36",
              "type_37",
              "type_38",
              "type_39",
              "type_4",
              "type_5",
              "type_6",
              "type_7",
              "type_8",
              "type_9",
          ],
          [
              "class_1",
              "class_2",
              "class_3",
              "class_4",
              "class_5",
              "class_6",
              "class_7",
          ],
      ]
      assert cov_type.nlevels() == [
          0,
          0,
          0,
          0,
          0,
          0,
          0,
          0,
          0,
          0,
          4,
          40,
          7,
      ]
  
label_desc: Use image node_id 1 for calling a global function with example usage: levels_na() without return types