message: Minor fixes in test_ad.py
diff: @@ -363,6 +363,7 @@ def test22_scatter_fwd(m):
  ref = [3.0, 0.0, 32.0, 0.0, 48.0, 0.0, 64.0, 0.0, 0.0, 0.0]
  assert ek.allclose(buf, ref)
  + ek.set_grad(buf, 0.0)
  ek.forward(x)
  grad = ek.grad(buf)
  @@ -671,5 +672,5 @@ def test41_replace_grad(m):
  z2 = z*z
  ek.backward(z2)
  assert ek.allclose(z2, [1, 16, 81])
  - assert ek.width(ek.grad(x)) == 0
  + assert ek.grad(x) is None
  assert ek.allclose(ek.grad(y), [12, 32, 36])
  