Add GCN and Node2VecGCN models

[tizianocitro]

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Description

• Added GCN model
• Added Node2VecGCN model
• Added examples

Checklist

• Does your submission pass all tests? (use make test)
• Have you written tests to cover all your changes? If not, provide a reason.
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• Have you added an explanation of what your changes are and why you'd like us to include them?