Add warning about StringLookup issue in TF 2.8.0

13_loading_and_preprocessing_data.ipynb

@@ -2650,6 +2650,13 @@
     "embedding_layer(np.array([2, 4, 2]))"
    ]
   },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "**Warning**: there's currently a bug in TensorFlow 2.8.0 ([issue #54459](https://github.com/tensorflow/tensorflow/issues/54459)) which prevents using a `StringLookup` layer inside a model, so the next 4 cells will fail. If you run into this issue, you must either skip these cells, or downgrade to TensorFlow 2.7.1 (e.g., using `%pip install tensorflow~=2.7.1`) and run the notebook again. I'll update this notebook as soon as the bug is fixed, or when a better workaround exists."
+   ]
+  },
   {
    "cell_type": "code",
    "execution_count": 97,