Follow-ups from gh-21150:

• ENH: ndimage: add array API standard support #21150 (comment) (TST: ndimage: unskip test_boundary_spline_accuracy for torch #21282)
• ENH: ndimage: delegate to CuPy #21091 (comment): We should try to add an @xfail_xp_backends decorator, to match @skip_xp_backends. (TST: add xfail_xp_backends #21334)
• ENH: ndimage: add array API standard support #21150 (comment): Tests for array-likes should be added throughout.
• ENH: ndimage: add array API standard support #21150 (comment): We should try to add the ability to say "only run for these backends" to the skip infra. (done in TST: fix GPU failures #21294)
• ENH: ndimage: add array API standard support #21150 (comment): Some functions produce object arrays which are incompatible with alternative backends. We probably want to deprecate and return regular arrays instead.