Sync NEON optimized kernels to current upstream PR (#19119)

[jgibson2]

Summary

Replaces the in-tree NEON grid_sampler_2d / sum implementation on polycam with the version that's been through three rounds of review on pytorch/executorch#19119. No new functionality — purely catching us up to the version that's about to land upstream.

What changed vs the version on polycam

1. Hardware fp16 vs software fp16 split. The original polycam version compiled the entire op_grid_sampler_2d.cpp with -march=armv8.2-a+fp16, which raises SIGILL on ARMv8 chips that lack the +fp16 extension. The new version moves the HW-fp16 path into a dedicated TU (op_grid_sampler_2d_fp16_hw.cpp + .h) compiled as a separate buck cxx_library / CMake OBJECT library so the -march flag stays scoped to that one TU. The dispatcher in op_grid_sampler_2d.cpp picks between the HW path and a software-convert NEON path at runtime via cpuinfo_has_arm_neon_fp16(). Plain ARMv8 chips run the SW path with no illegal instructions; ARMv8.2+ chips get the faster HW path.

2. Forward declaration of grid_sampler_2d_bilinear_fp16_hw extracted into op_grid_sampler_2d_fp16_hw.h so the definition site has a visible prototype (fixes -Wmissing-prototypes).

3. dtypes_match gate at the top of the fast-path eligibility check (input.scalar_type() == grid.scalar_type() == out.scalar_type()). The reinterpret_cast<__fp16*> in the HW kernel and the data_ptr<T>() calls in the other paths each assume a single dtype across all three tensors; mismatched buffers would silently corrupt output. Falls back to portable when types disagree.

CMake side switches from set_source_files_properties on a single source to a dedicated OBJECT library (grid_sampler_2d_fp16_hw_impl) linked into optimized_kernels via $<BUILD_LOCAL_INTERFACE:>. Mirrors the buck cxx_library of the same name.

What is NOT touched

• kernels/optimized/verify.cpp — polycam-specific on-device verification binary, stays as-is.
• tools/cmake/common/preset.cmake — polycam's DESCRIPTION quoting fix, stays as-is.

Test plan

• CMake build clean: cmake --build cmake-out --target optimized_kernels (macOS arm64, EXECUTORCH_BUILD_KERNELS_OPTIMIZED=ON).
• op_grid_sampler_2d_fp16_hw.cpp.o archived into liboptimized_kernels.a (verified via ar -t).
• Re-run the on-device verify binary (verify_optimized_kernels) on a Pixel 9 to confirm fp16 HW path still produces matching outputs.
• Re-run the depth model end-to-end to confirm latency unchanged.

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