class_docstr: DayCumsum Operator during start time and end time.
  
  Parameters
  ----------
  feature : Expression
      feature instance
  start : str
      the start time of backtest in one day.
      !!!NOTE: "9:30" means the time period of (9:30, 9:31) is in transaction.
  end : str
      the end time of backtest in one day.
      !!!NOTE: "14:59" means the time period of (14:59, 15:00) is in transaction,
              but (15:00, 15:01) is not.
      So start="9:30" and end="14:59" means trading all day.
  
  Returns
  ----------
  feature:
      a series of that each value equals the cumsum value during start time and end time.
      Otherwise, the value is zero.
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code: def period_cusum(self, df):
      df = df.copy()
      assert len(df) == 240 // self.data_granularity
      df.iloc[0 : self.start_id] = 0
      df = df.cumsum()
      df.iloc[self.end_id + 1 : 240 // self.data_granularity] = 0
      return df
  
imports: import numpy
  import pandas
  from datetime import datetime
  from qlib.data.cache import H
  from qlib.data.data import Cal
  from qlib.data.ops import ElemOperator, PairOperator
  from qlib.utils.time import time_to_day_index
label_desc: Use image node_id 2 for calling the DayCumsum obj's underlying member method code with example usage: obj.period_cusum(df) and returns: df