TorchVision: Add models with pre-trained weights.

[NiklasGustafsson]

We should add the following models from TorchVision, with the same pre-loaded weights:

https://pytorch.org/vision/stable/models.html#

See #586 for the origin story.

[NiklasGustafsson]

Tracking: added torchvision models for ResNet, AlexNet, and VGG, all of which allow for "manual" loading of pre-trained weights. Instructions for doing so are in the doc comments.

[yueyinqiu]

#713 (comment)