Add GNNPP model

[saaahahaa-cyber]

Description

Add GNNPP model and GNNPP convolution layer to GammaGL.

Checklist

• The PR title starts with [$CATEGORY] (such as [NN], [Model], [Doc], [Feature])
• Changes are complete (i.e. I finished coding on this PR)
• All changes have test coverage
• Code is well-documented
• To the best of my knowledge, examples are either not affected by this change, or have been fixed to be compatible with this change
• Related issue is referred in this PR

Changes

• Add GNNPP model
• Add GNNPP convolution layer
• Add GNNPP example and README
• Add test for GNNPP convolution layer