Created by: zhaoyuchen2018

Add descending parameter for argsort per vis requirement。This is a flag with bool datatype. Default value is false. If descending=True, will sort by descending order, else by ascending order. test=develop

chinese doc: https://github.com/PaddlePaddle/FluidDoc/pull/1621 Signed-off-by: zhaoyuchen zhaoyuchen01@baidu.com