Segmentation Models - v0.1.3

Updates

• New architecture Unet++ (#279)
• New encoders RegNet, ResNest, SK-Net, Res2Net (#286)
• Updated timm version (0.3.2)
• Improved docstrings and typehints for models
• Project documentation on https://smp.readthedocs.io

Thanks to @azkalot1 for the new encoders and architecture!