class_docstr: Desc: Data preprocessing,
        Here, e.g.    15 days for training,
                      3 days for validation,
                      and 6 days for testing
code: def __len__(self):
      return self.data_x.shape[1] - self.input_len - self.output_len + 1
  
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 4 for calling the PGL4WPFDataset obj's underlying member method code with example usage: obj.__len__() and returns: unknown