fix bug in optimizer epoch counter
fix bug in optimizer epoch counter

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