Add ROCm/HIP compatibility to CuMesh, enabling all 3 extensions

[ZJLi2013]

Add ROCm/HIP compatibility to CuMesh, enabling all 3 extensions (cumesh._C, cumesh._cubvh, cumesh._cumesh_xatlas) to compile and run on AMD GPUs.

Changes (4 files)

• src/atlas.cu: Replace cuda::std::plus() with portable cub::Sum() — the CCCL cuda::std header is not available on HIP
• src/clean_up.cu: Use rocprim::tuple for int3_decomposer on HIP via #ifdef — cuda::std::tuple and thrust::tuple are both unavailable/broken on ROCm 6.4
• src/dtypes.cuh: Add __host__ qualifier to all Vec3f and QEM methods — hipCUB's DeviceSegmentedReduce template instantiation requires host-callable constructors
• setup.py: Guard cubvh-specific nvcc flags (--extended-lambda, -U__CUDA_NO_HALF_*) behind IS_HIP check — these flags are NVIDIA-specific and cause errors with hipcc

What works

• All CUB device algorithms (RadixSort, Scan, Select, Reduce, ReduceByKey, SegmentedReduce) are automatically converted to hipCUB by PyTorch's hipify
• All CUDA runtime APIs (cudaMalloc, cudaMemcpy, etc.) are automatically converted to HIP equivalents
• cubvh submodule (half precision + Eigen) compiles without additional changes
• xatlas (CPU-only) is unaffected

What is NOT covered

• No functional changes to CUDA code paths — all changes are additive #ifdef guards or portable replacements
• No new dependencies introduced

Usage on AMD GPUs

git clone --recursive https://github.com/JeffreyXiang/CuMesh.git
cd CuMesh
GPU_ARCHS=gfx942 pip install . --no-build-isolation

Test Environment

GPU: AMD Instinct MI300X (gfx942)
ROCm: 6.4.3
PyTorch: 2.6.0
Docker: rocm/pytorch:rocm6.4.3_ubuntu24.04_py3.12_pytorch_release_2.6.0

Test	Status
Compile all 3 extensions (_C, _cubvh, _xatlas)	✅
Import all modules	✅
Mesh init + read (vertex/face roundtrip)	✅
Compute face normals	✅
Compute vertex normals	✅
Mesh simplification (4→2 faces)	✅
Remove duplicate faces	✅