Fix/qdq optional zero point input (#32051)
This pull request enhances the handling of optional inputs for quantized
operators, ensuring correctness when optional zero point inputs are
omitted and improving code robustness. The most important changes are
grouped below:
**Core Functionality Improvements:**
* Added a new utility function `SetOptionalInput` in `s8_to_u8.cc` to
correctly set optional input slots in a node, updating both input
definitions and input argument counts, and handling omitted inputs
gracefully.
[[1]](diffhunk://#diff-f1ed8d1a3ceb3229c24831a98c511aa862d535a8ba3c486bd7ca47a2244b4fe4R8-R32)
[[2]](diffhunk://#diff-b6b2ee740008dfb1616c212a5fe976a30a489e83e7da3f5389d63487025fc0f7R78-R79)
* Updated logic in both `TryConvertDynamicQuantizeLSTM` and
`ConvertS8WeightToU8` to use `SetOptionalInput` when assigning new
initializers for zero point inputs, ensuring proper handling of optional
inputs.
[[1]](diffhunk://#diff-9082e1a8710cae2bf43315ecca905dc65fb8e13956287506b7f71134c777803bL140-R141)
[[2]](diffhunk://#diff-9082e1a8710cae2bf43315ecca905dc65fb8e13956287506b7f71134c777803bL151-R153)
[[3]](diffhunk://#diff-f1ed8d1a3ceb3229c24831a98c511aa862d535a8ba3c486bd7ca47a2244b4fe4L51-R77)
**Bug Fixes and Robustness:**
* Modified checks for zero point input existence to require both
non-null and `.Exists()`, preventing errors when optional inputs are
omitted.
[[1]](diffhunk://#diff-9082e1a8710cae2bf43315ecca905dc65fb8e13956287506b7f71134c777803bL76-R76)
[[2]](diffhunk://#diff-9082e1a8710cae2bf43315ecca905dc65fb8e13956287506b7f71134c777803bL88-R90)
[[3]](diffhunk://#diff-f1ed8d1a3ceb3229c24831a98c511aa862d535a8ba3c486bd7ca47a2244b4fe4L29-R54)
**Testing Improvements:**
* Added a new unit test `MatMulIntegerOmittedZeroPoints` to verify that
the transformer correctly handles omitted zero point inputs, fills in
optional input slots, and generates the expected uint8 initializer.
