[Bugfix] Fix PyTorch stable ABI compatibility for permute_cols

[kilork]

Purpose

Fix build failure when combining PR vllm-project#37491 (CUTLASS upgrade to v4.4.2) with commit 8b10e4f (Migrate permute_cols to libtorch stable ABI). The build fails with:

static_assert(std::is_trivially_copyable_v<T>);
error: non-static data member 'torch::stable::detail::ToImpl<const torch::stable::Tensor&>::call(...)::Result::t' in a union may not have reference type

Root Cause

The PyTorch Stable ABI (introduced in PyTorch 2.10) requires all types to be trivially copyable for serialization between the C-shim and custom ops. Reference types (const Tensor&) are not trivially copyable and cannot be used as function parameters in STABLE_TORCH_LIBRARY registrations.

The code in csrc/libtorch_stable/ops.h and csrc/libtorch_stable/permute_cols.cu uses:

torch::stable::Tensor permute_cols(torch::stable::Tensor const& A,
                                   torch::stable::Tensor const& perm);

This is incompatible with the Stable ABI requirement documented at https://pytorch.org/docs/stable/library.html (see "Stable C++ ABI" / torch::stable namespace).

Fix

Change function signatures from pass-by-reference to pass-by-value:

csrc/libtorch_stable/ops.h:7-8

// Before:
torch::stable::Tensor permute_cols(torch::stable::Tensor const& A,
                                   torch::stable::Tensor const& perm);

// After:
torch::stable::Tensor permute_cols(torch::stable::Tensor A,
                                   torch::stable::Tensor perm);

csrc/libtorch_stable/permute_cols.cu:70-71

// Before:
torch::stable::Tensor permute_cols(torch::stable::Tensor const& A,
                                   torch::stable::Tensor const& perm) {

// After:
torch::stable::Tensor permute_cols(torch::stable::Tensor A,
                                   torch::stable::Tensor perm) {

Additional Fix

Also added missing CUTLASS include directories to _C_stable_libtorch target in CMakeLists.txt:1015-1016 (matching what _C already has).

Test Plan

Build vllm with both PR vllm-project#37491 applied and commit 8b10e4f present:

VLLM_USE_PRECOMPILED=1 uv pip install -e .

Test Result

Build completes successfully on CUDA.

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• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
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Co-authored-by: Claude noreply@anthropic.com