[Model] Add link prediction models.

[baoleai]

🚀 The feature, motivation and pitch

GLT has provided both stand-alone and distributed examples of node classification models, but examples of link prediction models are not yet available. So we should add link prediction GNN examples based on LinkNeighborLoader and DistLinkNeighborLoader like PyG's. in detail, these tasks are as follows

• homo GraphSAGE refer to https://github.com/pyg-team/pytorch_geometric/blob/master/examples/graph_sage_unsup_ppi.py
• hetero GraphSAGE
• docs https://github.com/alibaba/graphlearn-for-pytorch/blob/main/docs/get_started/link_pred.md

Alternatives

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Additional context

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