[Quant tool] Extend support for QDQ type conversion at graph output (#20841)
### Description
Allows mixed-precision overrides that adds a QDQ quantization type
conversion sequence at a graph output that **is not** consumed by other
nodes. This is not a common use-case but should handle it instead of
raising an error.
#### Example
Original model
mixed-precision overrides:
```python
mixed_prec_overrides = {
"input_0": [{"quant_type": QuantType.QUInt16}],
"op_0_out": [
{
"quant_type": QuantType.QUInt16,
"convert": {"quant_type": QuantType.QUInt8},
}
],
}
quantize_static(
float_model_path,
qdq_model_path,
data_reader,
quant_format=QuantFormat.QDQ,
activation_type=QuantType.QUInt8,
op_types_to_quantize=[node.op_type for node in float_model.graph.node],
extra_options={
"TensorQuantOverrides": mixed_prec_overrides,
},
)
```
QDQ model:
### Motivation and Context
This scenario is arising for certain quantization configurations. Should
handle it gracefully.
onnxruntime
3044aa87 - [Quant tool] Extend support for QDQ type conversion at graph output (#20841)
