Add Leiden to Notebook (rapidsai#152)

Part of rapidsai#123 

Building off of rapidsai#150

Authors:
  - Ralph Liu (https://github.com/nv-rliu)
  - Rick Ratzel (https://github.com/rlratzel)

Approvers:
  - Rick Ratzel (https://github.com/rlratzel)

URL: rapidsai#152

Author: nv-rliu

notebooks/demo/nx_cugraph_demo_2506.ipynb

@@ -496,6 +496,63 @@
     "plot_hist(bc, \"betweenness centrality\")"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "## Backend-only Functions\n",
+    "Besides improved performance, another benefit backends provide is the ability to add functionality to NetworkX that is not present in the default implementation.\n",
+    "\n",
+    "NetworkX 3.5 adds the `leiden_communities` function, but still does not provide an implementation. This allows backends to implement Leiden community detection using a common function signature, so when other backends or even NetworkX provide an implemtation, users can use those without requiring code changes.\n",
+    "\n",
+    "### Leiden community detection\n",
+    "Let's take a look at the communities in the Pokec social network dataset using `leiden_communities`"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "from networkx.algorithms.community import leiden_communities"
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "%%time\n",
+    "leiden_res = leiden_communities(G, seed=42, backend=\"cugraph\")"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "`leiden_communities` returns a list of sets, where each set contains the node IDs making up a community in the graph."
+   ]
+  },
+  {
+   "cell_type": "code",
+   "execution_count": null,
+   "metadata": {},
+   "outputs": [],
+   "source": [
+    "print(f\"Total number of extracted communities: {len(leiden_res)}\")\n",
+    "\n",
+    "sizes = [len(s) for s in leiden_res]\n",
+    "\n",
+    "print(f\"Largest community: {max(sizes)}\")\n",
+    "print(f\"Smallest community: {min(sizes)}\")\n",
+    "\n",
+    "import statistics\n",
+    "\n",
+    "print(f\"Median community size: {statistics.median(sizes)}\")"
+   ]
+  },
   {
    "cell_type": "markdown",
    "metadata": {
@@ -506,9 +563,21 @@
     "\n",
     "The [Facebook Network Analysis](https://networkx.org/nx-guides/content/exploratory_notebooks/facebook_notebook.html)\n",
     "example in [nx-guides](https://networkx.org/nx-guides) goes much more in depth\n",
-    "and is a good tour of networkx analysis.\n",
+    "and is a good tour of networkx analysis."
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "---\n",
+    "\n",
+    "### Information on the Pocek Social Network dataset used in this notebook\n",
     "\n",
-    "TODO: mention leiden, FA2, give suggestions and warnings"
+    "**Authors:** Lubos Takac and Michal Zabovsky  \n",
+    "**Title:** SNAP Datasets, Stanford Large Network Dataset Collection  \n",
+    "**URL:** [http://snap.stanford.edu/data](http://snap.stanford.edu/data)  \n",
+    "**Date:** May 2012"
    ]
   }
  ],