code: def get_length(self, y):
          
          lens = [self.find_pad_index(row) for row in y]
          return lens
docs: Get true length of y.
  
          Args:
              y (list): padded list.
  
          Returns:
              lens: true length of y.
  
          Examples:
              >>> y = [[1, 0, 0], [1, 1, 0], [1, 1, 1]]
              >>> self.get_length(y)
              [1, 2, 3]