fix GPU data race
fix GPU data race

Previously, the prefetch GPU -> top GPU and prefetch CPU -> prefetch GPU
copies were launched concurrently in separate streams, allowing the next
batch to be copied in before the current one is read.

This patch explicitly synchronizes the prefetch -> top copy wrt the
host, preventing the CPU -> GPU from being launched until its
completion.

tackgeun/caffe,longjon/caffe,gnina/gnina,tackgeun/caffe,CZCV/s-dilation-caffe,gnina/gnina,longjon/caffe,wangg12/caffe,wangg12/caffe,wangg12/caffe,tackgeun/caffe,longjon/caffe,gnina/gnina,gnina/gnina,wangg12/caffe,CZCV/s-dilation-caffe,gnina/gnina,longjon/caffe,tackgeun/caffe,CZCV/s-dilation-caffe,gnina/gnina,CZCV/s-dilation-caffe