Fix forked test-suite segfaults (OpenMP after fork)

[sbryngelson]

Problem

The two grad-vs-torch CIFAR tests (test_conv2d_pad_grad_matches_torch, test_group_norm_train_grad_matches_torch) crashed with SIGSEGV only late in the full pytest tests/ run, yet passed in isolation — so it looked environmental.

Root cause

The macOS crash report pinned the faulting thread entirely in libomp.dylib — not the ANE/Espresso/Metal stack:

libomp.dylib  __kmp_suspend_initialize_thread
libomp.dylib  __kmp_fork_barrier(int, int)
libomp.dylib  __kmp_launch_worker(void*)
EXC_BAD_ACCESS (SIGSEGV) KERN_INVALID_ADDRESS at 0x580

This is the classic OpenMP-thread-pool-after-fork() failure:

1. numpy/torch start an OpenMP worker-thread pool in the parent pytest process.
2. pytest-forked calls os.fork(); fork() does not copy the worker threads, so the child inherits a pool with dead thread descriptors.
3. The child's next OpenMP op — torch CPU autograd in these tests — calls into libomp (__kmp_fork_barrier), dereferences a dead descriptor, and segfaults.

That explains both symptoms: it only triggers once enough preceding tests have initialized the pool in the parent, and it's about threads, not test size (the tests are tiny).

Fix

Force OpenMP/BLAS single-threaded in tests/conftest.py, before any test imports numpy/torch:

for _thread_var in ("OMP_NUM_THREADS", "MKL_NUM_THREADS",
                    "OPENBLAS_NUM_THREADS", "VECLIB_MAXIMUM_THREADS"):
    os.environ.setdefault(_thread_var, "1")

With one OpenMP thread there is no worker pool to break across fork(), so the child is fork-safe. setdefault lets a caller override. These tiny test ops gain nothing from OpenMP parallelism, so there's no meaningful slowdown.

Verification

• The 9-file subset that reliably reproduced the crash: 224 passed, 0 segfaults.
• Full forked suite: 527 passed in 497.78s, 0 crashes / 0 failures (was 2 SIGSEGV), with the env set only by conftest.py.

Independent of the cleanup in #3.

[sbryngelson]

Folded into #3 (squashed into a single commit with the test/bench cleanup, per request).