code: def _transform_col(self, x, i):
          
          return x.fillna(NAN_INT).map(self.target_encoders[i]).fillna(self.target_mean)
docs: Encode one categorical column into average target values.
          Args:
              x (pandas.Series): a categorical column to encode
              i (int): column index
          Returns:
              x (pandas.Series): a column with labels.