GraphEdgeRef
data class GraphEdgeRef(val id: String, val sourceId: String, val destinationId: String, val sourceOutputIndex: Int = 0, val destinationInputIndex: Int = 0, val tensorSpec: TensorSpec, val metadata: Map<String, Any> = emptyMap())(source)
Lightweight version of GraphEdge that only stores node IDs Useful for serialization and when nodes are stored separately