TestAutograd::test_grad_ids_pre_post passes spuriously #2898

The test:

is supposed to check that passing a container to mx.grad doesn't mutate the items in the container. As noted in the docs for id(), two objects may have the same ids if they are never alive at the same time.

If I modify the test slightly:

import mlx.core as mx

def fun(arrs):
    return arrs[0]

arrs = [mx.array(1.0)]
arr = arrs[0]
mx.grad(fun)(arrs)
assert arr is arrs[0]

That fails. I think that means the test just happens to pass because of re-used ids.

This means #1854 didn't fix the issue the test is testing for, despite the guesswork here.

Originally posted by @ngoldbaum in #2807