PyTorch lands XNNPACK update f6486e3e1d, fixes Vulkan conv2d tests

PyTorch landed PR #191206, which updates XNNPACK to GitHub revision f6486e3e1d. This is the third attempt at this change, hence the "Re-re-re-land" tag, and it finally passed CI. The PR, linked to Executorch PR #21399, also updates the Buck build configuration. A key part of the change fixes Vulkan backend tests that were failing with the XNNPACK bump, specifically the conv2d_gemm_dynamic_test. This test validates dynamic convolution operations using GEMM (general matrix multiplication) on Vulkan, a cross-platform GPU API commonly used on Android devices.

XNNPACK is a highly optimized library for neural network inference on ARM and x86 CPUs, often used in mobile and edge deployments. By updating it, PyTorch ensures better performance and correctness for models running through Executorch, its lightweight runtime for on-device AI. The fix for the Vulkan test is particularly important for developers targeting Android GPUs, as it resolves a potential regression in convolution performance. Overall, this merge tightens the integration between PyTorch's core and its mobile inference stack, promising smoother and faster AI execution on edge devices.