Update function doc to use public placeholder_value() API for TraceTypes.

PiperOrigin-RevId: 515666479

Author: MarkDaoust

site/en/guide/function.ipynb

@@ -572,16 +572,23 @@
         "# can have significant performance benefits.\n",
         "\n",
         "class FruitTraceType(tf.types.experimental.TraceType):\n",
-        "  def __init__(self, fruit_type):\n",
-        "    self.fruit_type = fruit_type\n",
+        "  def __init__(self, fruit):\n",
+        "    self.fruit_type = type(fruit)\n",
+        "    self.fruit_value = fruit\n",
         "\n",
         "  def is_subtype_of(self, other):\n",
+        "      # True if self subtypes `other` and `other`'s type matches FruitTraceType.\n",
         "      return (type(other) is FruitTraceType and\n",
         "              self.fruit_type is other.fruit_type)\n",
         "\n",
         "  def most_specific_common_supertype(self, others):\n",
+        "      # `self` is the specific common supertype if all input types match it.\n",
         "      return self if all(self == other for other in others) else None\n",
         "\n",
+        "  def placeholder_value(self, placeholder_context=None):\n",
+        "      # Use the fruit itself instead of the type for correct tracing.\n",
+        "      return self.fruit_value\n",
+        "\n",
         "  def __eq__(self, other):\n",
         "    return type(other) is FruitTraceType and self.fruit_type == other.fruit_type\n",
         "  \n",
@@ -591,7 +598,7 @@
         "class FruitWithTraceType:\n",
         "\n",
         "  def __tf_tracing_type__(self, context):\n",
-        "    return FruitTraceType(type(self))\n",
+        "    return FruitTraceType(self)\n",
         "\n",
         "class AppleWithTraceType(FruitWithTraceType):\n",
         "  flavor = tf.constant([1, 2])\n",
@@ -1831,8 +1838,8 @@
   ],
   "metadata": {
     "colab": {
-      "collapsed_sections": [],
       "name": "function.ipynb",
+      "provenance": [],
       "toc_visible": true
     },
     "kernelspec": {