Registration tutorial to use MONAI_DATA_DIRECTORY (#668)

* Registration tutorial to use MONAI_DATA_DIRECTORY

Author: rijobro

2d_registration/registration_mednist.ipynb

@@ -119,12 +119,36 @@
     "import torch\n",
     "from torch.nn import MSELoss\n",
     "import matplotlib.pyplot as plt\n",
+    "import os\n",
+    "import tempfile\n",
     "\n",
     "\n",
     "print_config()\n",
     "set_determinism(42)"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Setup data directory\n",
+    "\n",
+    "You can specify a directory with the `MONAI_DATA_DIRECTORY` environment variable.  \n",
+    "This allows you to save results and reuse downloads.  \n",
+    "If not specified a temporary directory will be used."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "directory = os.environ.get(\"MONAI_DATA_DIRECTORY\")\n",
+    "root_dir = tempfile.mkdtemp() if directory is None else directory\n",
+    "print(root_dir)"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {
@@ -188,7 +212,7 @@
     }
    ],
    "source": [
-    "train_data = MedNISTDataset(root_dir=\"./\", section=\"training\", download=True, transform=None)\n",
+    "train_data = MedNISTDataset(root_dir=root_dir, section=\"training\", download=True, transform=None)\n",
     "training_datadict = [\n",
     "    {\"fixed_hand\": item[\"image\"], \"moving_hand\": item[\"image\"]}\n",
     "    for item in train_data.data if item[\"label\"] == 4  # label 4 is for xray hands\n",