Atapt Node and GraphView class to cyclic graph

This issue is a more focused discussion about a specific memory leak among those presented in aidge#195

Context

• Cyclic graph -> indirect leak due to shared_ptr reference between Nodes.

Current reference system

%%{init: {'flowchart': { 'curve': 'monotoneY'}, 'fontFamily': 'Verdana' } }%%
flowchart TB

ConvOp{{"Conv<br/>(Operator)"}}:::earth_yellow
ConvNode(("Conv<br/>(Node)")):::earth_blue

WeightOp{{"Producer<br/>(Operator)"}}:::earth_yellow
WeightNode(("Producer<br/>(Node)")):::earth_blue

BiasOp{{"Producer<br/>(Operator)"}}:::earth_yellow
BiasNode(("Producer<br/>(Node)")):::earth_blue


subgraph "User Scope"
    ConvNode
end

ConvNode --> ConvOp
WeightNode --> WeightOp
BiasNode --> BiasOp

WeightNode & BiasNode -..->|"weak_ptr"| ConvNode
ConvNode --->|"shared_ptr"| WeightNode & BiasNode

classDef earth_yellow fill:#F0D760,stroke:#333,stroke-width:2px;
classDef earth_blue fill:#B3E5FC,stroke:#333,stroke-width:2px;
linkStyle 3,4 stroke:#000,stroke-width:2px,stroke-dasharray: 5 5;
linkStyle 0,1,2,5,6 stroke:#ff0000,stroke-width:2px;

Solution

1. Nodes reference each other with a weak_ptr connection (there is no real choice for this...)
2. a global view references each created Node. This view is not known to the user. The purpose of such view is to prevent Nodes not referenced in the local scope from being deleted even though they are used (like Producers created for Conv)

When a Node is removed from a GraphView, if this Node has no other connection than the one to the GlobalView, it is deleted. Of course if the Node is in the current context, there is at least one reference other than the one from the GlobalView so it won't be deleted.

If a Nodes becomes unused in a function but is still referenced in the context of this function, I am not sure how to delete this Node when exiting the function.

Here would be the connections:

%%{init: {'flowchart': { 'curve': 'monotoneY'}, 'fontFamily': 'Verdana' } }%%
flowchart LR

ConvOp{{"Conv<br/>(Operator)"}}:::earth_yellow
ConvNode(("Conv<br/>(Node)")):::earth_blue

WeightOp{{"Producer<br/>(Operator)"}}:::earth_yellow
WeightNode(("Producer<br/>(Node)")):::earth_blue

BiasOp{{"Producer<br/>(Operator)"}}:::earth_yellow
BiasNode(("Producer<br/>(Node)")):::earth_blue

subgraph "GlobalView"
    subgraph "User Scope"
        ConvNode
    end
    WeightNode
    BiasNode
end

ConvNode --> ConvOp
WeightNode --> WeightOp
BiasNode --> BiasOp

WeightNode & BiasNode -..-> ConvNode
ConvNode <-..- WeightNode & BiasNode

classDef earth_yellow fill:#F0D760,stroke:#333,stroke-width:2px;
classDef earth_blue fill:#B3E5FC,stroke:#333,stroke-width:2px;
linkStyle 3,4,5,6 stroke:#000,stroke-width:2px,stroke-dasharray: 5 5;
linkStyle 0,1,2 stroke:#ff0000,stroke-width:2px;