sgl-kernel Metal kernels

Custom Apple Metal kernels for the MLX backend on Apple Silicon. Shader sources (*.metal) and C++ host / nanobind sources (*.cpp) in this directory are compiled by sgl-kernel/setup_metal.py into the sgl_kernel._metal extension and the sgl_metal_kernels.metallib archive, and exposed through Python wrappers in python/sgl_kernel/metal.py.

Kernels

Kernel	Description	Tested on
none yet	—	—

Adding a new Metal kernel

1. Add the shader under csrc/metal/<kernel>.metal.
2. Add the C++ host / nanobind binding under csrc/metal/<kernel>.cpp, exporting the entry point on the sgl_kernel._metal module.
3. Append both files to metal_shader_sources and cxx_sources in sgl-kernel/setup_metal.py.
4. Add a Python wrapper in python/sgl_kernel/metal.py that validates input shapes/dtypes and calls mx.eval on its operands before invoking the AOT C++ entry point.
5. Add a test under sgl-kernel/tests/ and update the Kernels table above with a short description and the hardware / OS / MLX version the kernel was validated on.

Note on placeholder.metal / placeholder.cpp

placeholder.metal and placeholder.cpp are intentionally empty. They exist only so that setup_metal.py has at least one shader source and one C++ source to compile, allowing the sgl_kernel._metal extension and the sgl_metal_kernels.metallib archive to build successfully before any real Metal kernels have been added. Both files (and their entries in metal_shader_sources / cxx_sources in setup_metal.py) MUST be removed once the first real kernel lands.