Fix: ci.py crash on macOS from duplicate libomp load

[ChaoWao]

Summary

On macOS, python ci.py -p a2a3sim (or a5sim) aborts every task with OMP: Error #15: Initializing libomp.dylib, but found libomp.dylib already initialized (SIGABRT) before any DeviceRunner code runs.

Root cause: Two distinct libomp.dylib copies get mapped into the single CI process:

• Homebrew's /opt/homebrew/opt/libomp/lib/libomp.dylib pulled in by numpy → openblas
• PyTorch's bundled .venv/.../torch/lib/libomp.dylib

They have different install names, so dyld loads both and Intel's libomp aborts on the second init. Surfaced after #493 collapsed sim CI into one long-lived Python process — now every golden's import numpy / import torch accumulates conflicting libomps in the same address space.

Changes

• ci.py: Set KMP_DUPLICATE_LIB_OK=TRUE at the top of the file on darwin, before any import that can transitively pull in numpy or torch. This is Intel's documented escape hatch; safe for our workload where numpy/torch are only used for golden reference math, not parallel OMP regions.
• docs/macos-libomp-collision.md (new): Full root cause analysis, debugging steps, reproducer, and explicit "what NOT to do" list so future contributors don't re-investigate the same rabbit hole. Linked from docs/ci.md.
• examples/a2a3/{aicpu,host}_build_graph/bgemm/golden.py: Rewrite the two remaining numpy-based goldens in torch for style consistency with the rest of examples/. Note this does not avoid the libomp collision on its own — import torch transitively imports numpy.

Also investigated: alternatives like ctypes.CDLL(..., RTLD_GLOBAL) pre-loading and DYLD_INSERT_LIBRARIES do not fix this, because the two dylibs have distinct LC_ID_DYLIB install names and dyld resolves dependencies by install name, not by symbol. See the doc for details.

Test plan

• python ci.py -p a2a3sim on macOS — 20/20 pass (previously 20/20 fail with SIGABRT)
• python ci.py -p a5sim on macOS — 12/12 pass (previously 12/12 fail with SIGABRT)
• python ci.py (both sims together) on macOS — 32/32 pass
• Linux sim CI still green (unchanged path — KMP_DUPLICATE_LIB_OK is only set on sys.platform == "darwin")

[gemini-code-assist]

Code Review

This pull request introduces a workaround for a libomp collision issue on macOS that causes SIGABRT when both numpy and torch are loaded in the same process. The fix involves setting KMP_DUPLICATE_LIB_OK=TRUE at the top of ci.py before other imports. Detailed documentation explaining the root cause and mitigation has been added in docs/macos-libomp-collision.md and referenced in docs/ci.md. Additionally, several golden reference scripts were updated to use torch instead of numpy for input generation and computation. I have no feedback to provide as there were no review comments to evaluate.