Fix GenerationConfig initialization in qnn_multimodal_runner

[kirklandsign]

Summary:
...changes.

• Replace fragile positional initialization with explicit field assignments
• Improves code clarity by making field names explicit
• Makes the code robust to future changes in GenerationConfig struct

Differential Revision: D105377713

[pytorch-bot]

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[Copilot]

Pull request overview

Switches the GenerationConfig aggregate-initializer in the QNN multimodal runner from positional to designated initializers so the call site is self-documenting and resilient to future field additions/reorderings in extension/llm/runner/irunner.h.

Changes:

• Replace positional braced initializer of GenerationConfig with C++20 designated initializers naming each field.

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[GregoryComer]

This is a C++20 feature, right? If it compiles, it's probably fine. I believe GCC/Clang support it as an extension on C++17 and we don't officially support MSVC + QNN at this point.

[kirklandsign]

This is a C++20 feature, right? If it compiles, it's probably fine. I believe GCC/Clang support it as an extension on C++17 and we don't officially support MSVC + QNN at this point.

GCC and Clang: These compilers have supported designated initializers as a "C extension" in C++ mode long before C++20. If you are using g++ or clang++ with -std=c++17, it will likely compile, though it might trigger a warning (e.g., -Wpedantic) saying "designated initializers are a C++20 extension."

MSVC (Visual Studio): Standard support started with MSVC 2019 v16.8. Earlier versions strictly following the C++17 standard will fail.