code: def insert_nans_and_zeros(ts_len: int) -> List:
      target = []
      for j in range(ts_len):
          # Place NaNs at even indices. Use convention no NaNs before start
          # date.
          if j != 0 and j % 2 == 0:
              target.append(np.nan)
          # Place zeros at odd indices
          else:
              target.append(0.0)
      return target
  
imports: import math
  import random
  from abc import abstractmethod
  from typing import Callable, Dict, List, NamedTuple, Optional, Tuple, Union, cast
  import numpy
  import pandas
  from pandas.tseries.frequencies import to_offset
  from pandas.tseries.offsets import BaseOffset, Week
  from gluonts.dataset.artificial.recipe import BinaryHolidays, BinaryMarkovChain, Constant, ForEachCat, Lag, LinearTrend, RandomCat, RandomGaussian, Stack, generate, take_as_list
  from gluonts.dataset.common import BasicFeatureInfo, CategoricalFeatureInfo, DataEntry, Dataset, ListDataset, MetaData, TrainDatasets
  from gluonts.dataset.field_names import FieldName
  from gluonts.dataset.stat import DatasetStatistics, calculate_dataset_statistics
label_desc: Use image node_id 7 for calling the ConstantDataset obj's underlying member method code with example usage: obj.insert_nans_and_zeros(ts_len) and returns: target