improve the speed of stacking the outputs
improve the speed of stacking the outputs

sxjscience/mxnet,sxjscience/mxnet,sxjscience/mxnet,sxjscience/mxnet,sxjscience/mxnet,sxjscience/mxnet,sxjscience/mxnet,sxjscience/mxnet,sxjscience/mxnet