PyTorch 0.4 support and new features

• PyTorch 0.4 support
• An implementation of Baidu's RNN pruning paper from ICLR 2017
  Narang, Sharan & Diamos, Gregory & Sengupta, Shubho & Elsen, Erich. (2017).
  Exploring Sparsity in Recurrent Neural Networks. (https://arxiv.org/abs/1704.05119)
• Add a word language model pruning example using AGP and Baidu RNN pruning
• Quantization aware training (4-bit quantization)
• New models: pre-activation ResNet for ImageNet and CIFAR, and AlexNet with batch-norm
• New quantization documentation content