pytorch DDP模式下, 获取数据的的preftech + stream

直接上代码
- DDP forward

if self.device_ids:
    if len(self.device_ids) == 1:
        inputs, kwargs = self.to_kwargs(inputs, kwargs, self.device_ids[0])
        output = self.module(*inputs[0], **kwargs[0])
    else:
        inputs, kwargs = self.scatter(inputs, kwargs, self.device_ids)
        outputs = self.parallel_apply(self._module_copies[:len(inputs)], inputs, kwargs)
        output = self.gather(outputs, self.output_device)
else:
    output = self.module(*inputs, **kwargs)

def scatter(self, inputs, kwargs, device_ids):
    return scatter_kwargs(inputs, kwargs, device_ids, dim=self.dim)

def scatter_kwargs(inputs, kwargs, target_gpus, dim