Add nested pmap image to demo notebook. (#2292)

cloud_tpu_colabs/NeurIPS_2019_JAX_demo.ipynb

@@ -876,6 +876,16 @@
       "execution_count": 0,
       "outputs": []
     },
+    {
+      "cell_type": "markdown",
+      "metadata": {
+        "id": "f-FBsWeo1AXE",
+        "colab_type": "text"
+      },
+      "source": [
+        "![nested pmap example](https://raw.githubusercontent.com/google/jax/master/cloud_tpu_colabs/images/nested_pmap.png)"
+      ]
+    },
     {
       "cell_type": "markdown",
       "metadata": {
@@ -1001,4 +1011,4 @@
       "outputs": []
     }
   ]
-}
+}