[VitisAI EP] Fix error in graph resolving (#26452)
### Description
<!-- Describe your changes. -->
Use the updated subgraph when cloning the model.
### Motivation and Context
When a node in model contains a subgraph attribute, onnxruntime will
optimize the subgraph. If the updated subgraph is not used in the cloned
model, it may cause errors during graph resolving.
