Add models and modelzoo, some bugfixes

New Features

• Add torchvision.models: Definitions and pre-trained models for common vision models
  • ResNet, AlexNet, VGG models added with downloadable pre-trained weights
• adding padding to RandomCrop. Also add transforms.Pad
• Add MNIST dataset

Performance Fixes

• Fixing performance of LSUN Dataset

Bug Fixes

• Some Python3 fixes
• Bug fixes in save_image, add single channel support