Adding Usage Examples for Graph Convolutional Layers

[rbSparky]

Currently, only a few convolutional layers have examples demonstrating how to use them, such as GCNConv, CGConv, MEGNetConv, GNNGraph, SGConv, and EGNNConv (6 out of 18 layers).

We should aim to provide examples for all layers.

Checklist

• [x] GCNConv
• [x] CGConv
• [x] MEGNetConv
• [x] GNNGraph
• [x] SGConv
• [x] EGNNConv
• [x] ChebConv
• [x] GraphConv
• [x] GATConv
• [x] GATv2Conv
• [x] GatedGraphConv
• [x] EdgeConv
• [x] GINConv
• [x] NNConv
• [x] SAGEConv
• [x] ResGatedGraphConv
• [x] AGNNConv
• [x] GMMConv
• [ ] TransformerConv PR #422