update notebook

Signed-off-by: Can-Zhao <volcanofly@gmail.com>

Author: Can-Zhao

generation/maisi/maisi_train_diff_unet_tutorial.ipynb

@@ -67,7 +67,7 @@
   },
   {
    "cell_type": "code",
-   "execution_count": 2,
+   "execution_count": null,
    "id": "e3bf0346",
    "metadata": {
     "execution": {
@@ -268,7 +268,6 @@
     "import subprocess\n",
     "from IPython.display import Image, display\n",
     "\n",
-    "from monai.apps import download_url\n",
     "from monai.data import create_test_image_3d\n",
     "from monai.config import print_config\n",
     "\n",