Add comment to reflect new gradient scaling interface

examples/0b_two_moons_backprop.ipynb

@@ -113,6 +113,7 @@
     "    with pxu.step(model):\n",
     "        (e, y_), g = pxf.value_and_grad(pxu.Mask(pxnn.LayerParam, [False, True]), has_aux=True)(loss)(x, y, model=model)\n",
     "    \n",
+    "    # Note there is not gradient scaling as we have already taken the mean of the loss, not its sum.\n",
     "    optim_w.step(model, g[\"model\"])"
    ]
   },