### Background
This change brings the capability to decompose a graph into sub-graphs. The objective of this feature is to allow a user to build a visually modular, and easier-to-understand graph. Also, allowing you to import a graph into your existing graph, without decluttering your existing graph.
This feature will require more implementation on the UI side, to allow the grouping of subgraph to be represented as a node in the builder.
### Changes 🏗️
Introduced a subgraph functionality with the following property:
* Sub-graph is simply a set of nodes that are grouped together, making it representable as a node.
* Sub-graph input & output pins/schema are the `InputBlock` / `OutputBlock` nodes present in the subgraph.
* The previous point implies that connecting two nodes from different sub-graphs, other than input/output nodes, is not allowed.
* Graph can be nested, but defined flatly, e.g.: graph is now only represented by three components: nodes, links, and subgraphs (a set of list of nodes). A nested subgraph is simply connecting a node inside a subgraph into another `InputBlock` node of another subgraph.
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