'Round robin the gpu device. (Reserve last gpu for expensive op).'

def _next_device(self):

    if (self._num_gpus == 0):
        return ''
    dev = ('/gpu:%d' % self._cur_gpu)
    if (self._num_gpus > 1):
        self._cur_gpu = ((self._cur_gpu + 1) % (self._num_gpus - 1))
    return dev
