fix USE_CPP=0 during local build

[vkuzo]

Summary:

Looks like this env var no longer matches its original intent (Python-only build)
as various other c++ extensions were added, fixing.

Test Plan:

with-proxy USE_CPP=0 pip install -e .

Reviewers:

Subscribers:

Tasks:

Tags:

[vkuzo]

Stack from ghstack (oldest at bottom):

• -> fix USE_CPP=0 during local build #1812

[pytorch-bot]

🔗 Helpful Links

🧪 See artifacts and rendered test results at hud.pytorch.org/pr/pytorch/ao/1812

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❌ 7 New Failures

As of commit f59eec9 with merge base 7963f9c ():

NEW FAILURES - The following jobs have failed:

• Build Linux Wheels / build / upload / manywheel-py3_9-cuda11_8 (gh)
• Build Linux Wheels / build / upload / manywheel-py3_9-cuda12_6 (gh)
  Unable to download artifact(s): Artifact not found for name: pytorch_ao__3.9_cu126_x86_64
• Build Linux Wheels / build / upload / manywheel-py3_9-cuda12_8 (gh)
• Run Regression Tests / test (CUDA 2.3, linux.g5.12xlarge.nvidia.gpu, torch==2.3.0, cuda, 12.1) / linux-job (gh)
  test/sparsity/test_marlin.py::SparseMarlin24::test_quant_sparse_marlin_layout_eager
• Run Regression Tests / test (CUDA 2.4, linux.g5.12xlarge.nvidia.gpu, torch==2.4.0, cuda, 12.1) / linux-job (gh)
  test/sparsity/test_marlin.py::SparseMarlin24::test_quant_sparse_marlin_layout_eager
• Run Regression Tests / test (CUDA 2.5.1, linux.g5.12xlarge.nvidia.gpu, torch==2.5.1 --index-url https://download.pytorch... / linux-job (gh)
  test/sparsity/test_sparse_api.py::TestQuantSemiSparse::test_sparse_marlin_compile_True
• Run Regression Tests / test-nightly (CUDA Nightly, linux.g5.12xlarge.nvidia.gpu, --pre torch==2.7.0.dev20250122 --index-... / linux-job (gh)
  test/sparsity/test_sparse_api.py::TestQuantSemiSparse::test_sparse_marlin_compile_True

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[vkuzo]

CI is failing, needs some work