CUDA graph phase N - graph updates

[leofang]

This task should cover updating both whole graphs and individual graph nodes.

[Andy-Jost]

Implementation plan for graph updates

Breaking this into five PRs:

1. Whole-graph update tests — Add tests for the existing Graph.update() API (cuGraphExecUpdate), which replaces an entire executable graph from a new graph definition. Refactoring: test file reorganization and breaking down of _graphdef.pyx into a submodule.

2. Edge mutation on GraphDef — Support adding and removing edges between nodes in a graph definition, wrapping cuGraphAddDependencies / cuGraphRemoveDependencies.

3. GraphBuilder interop with GraphDef — Add a method GraphBuilder.graph_def to get the explicit representation of a graph built by stream capture. This involves changing GraphBuilder to use an RAII graph handle internally so that ownership can be shared.

4. Node property setters (definition-level) — Add property setters to all GraphNode subclasses (KernelNode, MemsetNode, MemcpyNode, etc.) that call the corresponding cuGraph*NodeSetParams APIs to modify nodes in a graph definition.

5. Exec-level node updates — Enable modifying nodes in an instantiated graph using graph[node].property = new_value, so users can update kernel parameters, memset values, etc. without rebuilding the graph.