TST: add a test for torch.argsort defaulting to stable=True

cross-ref data-apis#356
which wrapped torch.argsort to fix the default, and
data-apis/array-api-tests#390
which made a matching change in the array-api-test suite.

Author: ev-br

tests/test_torch.py

@@ -130,3 +130,13 @@ def test_meshgrid():
     assert Y.shape == Y_ij.shape
     assert xp.all(Y == Y_ij)
 
+
+def test_argsort_stable():
+    """Verify that argsort defaults to a stable sort."""
+    # Bare pytorch defaults to an unstable sort, and the array_api_compat wrapper
+    # enforces the stable=True default.
+    # cf https://github.com/data-apis/array-api-compat/pull/356 and
+    # https://github.com/data-apis/array-api-tests/pull/390#issuecomment-3452868329
+
+    t = xp.zeros(50)    # should be >16
+    assert xp.all(xp.argsort(t) == xp.arange(50))