FIX: Use subprocess isolation for macOS torch tests instead of skipping

[Heyy-Himanshuu]

fixes #4075

On macOS, 17 torch-related tests across 5 files are completely skipped due to a known upstream PyTorch/OpenMP segfault (pytorch/pytorch#121101). When run without skipping, the tests either segfault or hang indefinitely, killing the entire CI suite.

This PR adds subprocess isolation to tests/conftest.py that:

• Detects skipif markers referencing GH TRACKER: MacOS segmentation fault for torch test #4075 at collection time via pytest_collection_modifyitems
• On macOS, replaces them with a subprocess_isolation marker
• Runs those tests in a fresh subprocess (subprocess.run) with a 5-minute timeout
• Reports segfaults/timeouts as FAILED with diagnostics instead of hanging the suite
• Uses an environment variable (_SHAP_SUBPROCESS_CHILD) to prevent recursion in the child process

No test files are modified — the hook intercepts markers dynamically at runtime. On Linux/Windows, the hooks are no-ops and tests run normally as before.

Why not pytest-forked?

pytest-forked uses os.fork(), which is unsafe with multithreaded C libraries on macOS and can make the problem worse.

How it works

Step	Parent process	Child subprocess
Collection	_SHAP_SUBPROCESS_CHILD unset → replaces skipif with subprocess_isolation	_SHAP_SUBPROCESS_CHILD=1 → removes skipif so test runs in-process
Execution	Hook wrapper spawns subprocess.run(...)	Test runs directly with clean OpenMP state
Recursion?	No — child has no subprocess_isolation marker	No — runs in-process normally

Checklist

• All pre-commit checks pass.
• Unit tests added (if fixing a bug or adding a new feature)

[CloseChoice]

Thanks for the PR and would be great if we could get the tests back to work, but I would prefer a decorator for the affected tests, something like:

@subprocess_isolated(reason="torch/OpenMP segfault on macOS, GH #4075")

The pytest magic is overly complex and error-prone.

[daidahao]

Thanks for the PR and would be great if we could get the tests back to work, but I would prefer a decorator for the affected tests, something like:

@subprocess_isolated(reason="torch/OpenMP segfault on macOS, GH #4075")

The pytest magic is overly complex and error-prone.

@CloseChoice Hi, I believe the failing macOS tests have been fixed by #4545.