class_docstr: Desc: Data preprocessing,
        Here, e.g.    15 days for training,
                      3 days for validation,
                      and 6 days for testing
code: def __getitem__(self, index):
      # Sliding window with the size of input_len + output_len
      s_begin = index
      s_end = s_begin + self.input_len
      r_begin = s_end
      r_end = r_begin + self.output_len
      seq_x = self.data_x[:, s_begin:s_end, :]
      seq_y = self.data_x[:, r_begin:r_end, :]
  
      if self.flag == "train":
          perm = np.arange(0, seq_x.shape[0])
          np.random.shuffle(perm)
          return seq_x[perm], seq_y[perm]
      else:
          return seq_x, seq_y
  
imports: import os
  import time
  import datetime
  import numpy
  import pandas
  import paddle
  from paddle.io import Dataset
  import pgl
  from pgl.utils.logger import log
label_desc: Use image node_id 3 for calling the PGL4WPFDataset obj's underlying member method code with example usage: obj.__getitem__(index) and returns: seq_x,  seq_y, seq_x,  seq_y