cuda.core: expose GraphDefinition cloning

Background

CUDA provides cuGraphClone() to create an independent copy of a graph definition and cuGraphNodeFindInClone() to map source nodes to their clones. cuda.core already handles CUDA-created clones internally when embedding child graphs, including copying and rekeying attachment metadata, but GraphDefinition does not expose cloning publicly.

Scope

Add a public cloning API for GraphDefinition, including:

• Clone the underlying CUgraph with cuGraphClone().
• Create an independent cuda.core graph hierarchy for the clone.
• Copy and rekey node attachment metadata across the complete hierarchy, including nested child graphs.
• Provide a supported way to map or reconstruct corresponding nodes in the clone.
• Preserve resource ownership and lifetime guarantees without coupling later source and clone mutations.
• Document the API and add release notes.

Acceptance criteria

• A graph definition can be cloned through a public cuda.core API.
• Source and clone can be mutated, instantiated, and destroyed independently.
• Nodes, edges, parameters, child graphs, and supported graph metadata are preserved.
• Attached resources remain alive independently for each graph and are released at the appropriate lifetime boundary.
• Tests cover node mapping, nested child graphs, attachments, mutation isolation, executable behavior, and cleanup.