训练一个深度学习检测模型，突然出现：

是因为next(batch_iterator)，可能迭代器读出来的数据为空。

 # load train data
 # 原先代码
 images, targets = next(batch_iterator)
 # 更改为：
 try:
     images, targets = next(batch_iterator)
 except:
     batch_iterator = iter(data_loader)
     images, targets = next(batch_iterator)