Solution 9 version 4 requires TF 2.2 (add warning)

16_nlp_with_rnns_and_attention.ipynb

@@ -2219,7 +2219,14 @@
    "cell_type": "markdown",
    "metadata": {},
    "source": [
-    "### Fourth version: using TF-Addons's seq2seq implementation with Teacher Forcing"
+    "### Fourth version: using TF-Addons's seq2seq implementation with a scheduled sampler"
+   ]
+  },
+  {
+   "cell_type": "markdown",
+   "metadata": {},
+   "source": [
+    "**Warning**: due to a TF bug, this version only works using TensorFlow 2.2."
    ]
   },
   {