Add PixelCNN++ improvements to GatedPixelCNN

[EugenHotaj]

Some things we could add:

• [ ] [easy] Use down/right shifted 3x3 conv instead of a right shifted 1x3 conv in the horizontal stack.
• [ ] [medium] Use a U-Net architecture with skip connections (this has shown improvement in some early experiments)
• [ ] [medium] [optional] Use gated ResNet layers in both the vertical and horizontal stacks instead of the adhoc GatedPixelCNNLayer.
• [ ] [hard] [optional] Implement the discretized logistic mixture for non-binary outputs

[EugenHotaj]

Unlikely to ever implement this.