kernel-builder: provide NCCL >= 2.32 headers and UCCL as build dependencies (for DeepEP) #949

Context

huggingface/kernels-community#1231 ports DeepEP 2.5.0 as kernels-community/deep-ep to support the upcoming deepep expert-parallel strategy in Transformers. Right now the port has to vendor some dependencies that kernel-builder should probably provide instead.

NCCL

• DeepEP 2.5's host code uses GIN and EFA-GDA APIs from NCCL 2.31/2.32.
• The torch that kernel-builder ships comes with NCCL 2.30.4. That's too old to build against.
• As a workaround, the port builds headers from an NCCL Git checkout pinned at 12df1a11afad322be5a204a2db890161cbf8131d (2.32.3) and uses them only at build time. The extension does not link or ship its own NCCL; it uses the symbols PyTorch has already loaded.
• Ask: make NCCL >= 2.32 headers available in kernel-builder, for example by bumping the NCCL that comes with torch or by adding NCCL as a build dependency in the Nix setup. Then the vendored headers can be removed from the deep-ep port.

UCCL (AMD / ROCm)

• We also want DeepEP on AMD GPUs, using the UCCL EP headers.
• Ask: check whether UCCL can be added as a kernel-builder dependency for ROCm builds. This needs Nix packaging.

Acceptance

• kernel-builder exposes NCCL >= 2.32 headers to CUDA kernels, so deep-ep can drop its vendored NCCL headers
• Decide whether UCCL should be a kernel-builder dependency for ROCm, and package it if so

Requested by Daniël de Kok - Slack thread - Agent trace