[QNN Quantization] Ensure fused nodes have names (#19650)
### Description
- Updates the `qnn_preprocess_model()` method to set a name for any new
nodes added to the graph (due to fusion).
- Updates the `qnn_preprocess_model()` method to set a name for any
unnamed nodes that previously existed in the original graph.
- Adds unit tests for fusions (previously missing)
- Checks that fused node names exist and are unique
- Checks that fused graph is equivalent to original graph
### Motivation and Context
Nodes are not strictly required to have names. However, a
planned/upcoming feature to support mixed-precision (integer) quantized
models needs nodes to have names.
