docs: def grouper_df(df, chunksize):
      """Evenly divide pd.DataFrame into n rows piece, no filled value 
      if sub dataframe's size smaller than n.
  
      :param df: ``pandas.DataFrame`` instance.
      :param chunksize: number of rows of each small DataFrame.
  
      **中文文档**
  
      将 ``pandas.DataFrame`` 分拆成等大小的小DataFrame。
      """
      data = list()
      counter = 0
      for tp in zip(*(l for col, l in df.iteritems())):
          counter += 1
          data.append(tp)
          if counter == chunksize:
              new_df = pd.DataFrame(data, columns=df.columns)
              yield new_df
              data = list()
              counter = 0
  
      if len(data) > 0:
          new_df = pd.DataFrame(data, columns=df.columns)
          yield new_df