Function validate_einsum_graph 

pub fn validate_einsum_graph(graph: &EinsumGraph) -> ValidationResult

Validate an EinsumGraph for structural integrity.

Runs all structural checks and returns a unified ValidationResult.

Checks performed

1. Empty graph (warning) – graph has zero nodes
2. Duplicate graph outputs (warning) – same tensor index listed twice
3. Node input references (error) – node inputs reference non-existent tensors
4. Node output references (error) – node outputs reference non-existent tensors
5. Unreachable nodes (warning) – nodes whose outputs are never consumed
6. Output references (error) – graph outputs reference non-existent tensors
7. Outputs without producer (error) – output tensors not produced by any node or input
8. Cycle detection (error) – data-flow cycles in the graph
9. Empty node outputs (error) – nodes that produce no tensors
10. Duplicate tensor names (info) – two tensors share the same name