Add support for building noarch kernels

[danieldk]

This change adds support for building noarch kernels. So far we have used the universal variant for kernels that do not have any AoT-compiled code. However, the universal variant has two important issues:

1. A kernel without AoT-compiled might still be backend-specific. E.g. NVIDIA CuTe-based kernels are not universal in the sense that they don't work on non-NVIDIA GPUs.
2. We cannot specify dependencies per backend.

To solve these issues, we introduce the noarch variants to replace universal kernels. Noarch kernels have variants of the shape torch-<backend> (e.g. torch-xpu). This resolves the issues outlined.

To support no-arch kernels, we update the build.toml format to v3, making the following changes:

• general.universal is removed.
• general.backends is introduced. This required option is used to list what backends the kernel supports.
• general.cuda-{minver,maxver} has been moved to the general.cuda section.

If a kernel supports backend X and has one or more kernels.* sections with backend = "X", then the kernel is an AoT-compiled kernel for that backend. Otherwise, it is a noarch kernel for that backend. Suppose that we have:

[general]
# ...
backends = ["cuda", "xpu"]
#...

[kernel.mykernel]
backend = "xpu"
# ...

then the XPU kernel will be AoT-compiled (e.g. build/torch29-cxx11-xpu20252-x86_64-linux), whereas the CUDA kernel will be noarch (torch-cuda).

An older build.toml can be updated automatically with build2cmake update-build build.toml.

[danieldk]

v3 is mostly a copy of v2, so I'll mark the changes here.

In general, universal was removed, backends was added.

[danieldk]

cuda-minver and cuda-maxver have moved from general to this new general.cuda section.

[danieldk]

I have moved generation of variant strings torch-... to the Torch derivation itself. This makes it easier to access it from everywhere.

[MekkCyber]

Yes makes sense

[MekkCyber]

Nice work! I didn’t follow every detail 😅, but the logic seems sound, Thank you

[MekkCyber]

can we add npu in the case of noarch variants ?

[MekkCyber]

Yes makes sense

[drbh]

changes look good to me!

small nit regarding the format of the config module, currently when adding a new version we need to make changes to the previous version files. it may be helpful to limit the version files to parsing toml only, and maintain a top level config that is populated by the different versions