fix bug: num_rois should be batch_images x rois_per_image
fix bug: num_rois should be batch_images x rois_per_image

TuSimple/mxnet,TuSimple/mxnet,TuSimple/mxnet,TuSimple/mxnet,TuSimple/mxnet,TuSimple/mxnet,TuSimple/mxnet,TuSimple/mxnet,TuSimple/mxnet,TuSimple/mxnet