torchvision是一个包，它服务于pytorch深度学习框架，用来生成图片，视频数据集，模型类和预训练的模型
torchvision由以下四个部分组成:
1. torchvision.datasets : Data loaders for popular vision datasets
2. torchvision.models : Definitions for popular model architectures, such as AlexNet, VGG, and ResNet and pre-trained models.
3. torchvision.transforms : Common image transformations such as random crop, rotations etc.
4. torchvision.utils : Useful stuff such as saving tensor (3 x H x W) as image to disk, given a mini-batch creating a grid of images, etc.