Add more featurizer options

[djsutherland]

• [x] torchvision's pretrained ResNets
• [x] torchvision's VGG
• [ ] Other torchvision models?
• [x] Identity featurizer to operate on pixels
• [x] mixtures of models, like in https://arxiv.org/abs/1904.02762
• [ ] autoencoders?
• [ ] gradient features from models, like https://arxiv.org/abs/1806.11006
• [ ] Laplacian pyramid features, like in https://arxiv.org/abs/1707.05776?