Add HNHN model

[tizianocitro]

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Description

• Added HNHN model
• Added node space setting (transductive or inductive) to split and enrich operations.
• Added number of nodes parameter to LPE node feature enricher to account for the possibility of nodes not appearing in the hyperedge index. This can happen in transductive case for example, where the train set contains all the nodes but may not have hyperedges with them in it.
• Fixed default learning rate for HGNN.

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