Generalize generate_model_outputs_using_core_keras_layers().

This change adds the following two new features to the above function:
(i) it supports nested custom layers of depth >2;
(ii) it allows the caller to exclude certain layers from the expansion.

Feature (ii) will be needed for the development of DP models that use
Trasformer or BERT-type layers.

PiperOrigin-RevId: 520919934

tensorflow_privacy/privacy/fast_gradient_clipping/gradient_clipping_utils.py

@@ -13,7 +13,7 @@
 # limitations under the License.
 """Utility functions that help in the computation of per-example gradient norms."""
 
-from typing import Any, Callable, Dict, Iterable, List, Optional, Text, Tuple, Union
+from typing import Any, Callable, Dict, Iterable, List, Optional, Set, Text, Tuple, Union
 
 from absl import logging
 import tensorflow as tf
@@ -36,19 +36,6 @@ def has_internal_compute_graph(input_object: Any):
   )
 
 
-def _get_internal_layers(
-    input_layer: tf.keras.layers.Layer,
-) -> List[tf.keras.layers.Layer]:
-  """Returns a list of layers that are nested within a given layer."""
-  internal_layers = []
-  if isinstance(input_layer, tf.keras.Model) and hasattr(input_layer, 'layers'):
-    for layer in input_layer.layers:
-      internal_layers.extend(_get_internal_layers(layer))
-  else:
-    internal_layers.append(input_layer)
-  return internal_layers
-
-
 def model_forward_pass(
     input_model: tf.keras.Model,
     inputs: PackedTensors,
@@ -114,18 +101,10 @@ def model_forward_pass(
         generator_outputs_list.extend(node_generator_outputs)
       else:
         # Otherwise, we parse the node directly.
-        node_layers = _get_internal_layers(node.layer)
-        for layer in node_layers:
-          node_layer_outputs, layer_generator_outputs = generator_fn(
-              layer, args, kwargs
-          )
-          generator_outputs_list.append(layer_generator_outputs)
-          args = (
-              node_layer_outputs
-              if isinstance(node_layer_outputs, tuple)
-              else (node_layer_outputs,)
-          )
-          kwargs = {}
+        node_layer_outputs, layer_generator_outputs = generator_fn(
+            node.layer, args, kwargs
+        )
+        generator_outputs_list.append(layer_generator_outputs)
 
       # Update the current dictionary of inputs for the next node.
       for x_id, y in zip(
@@ -163,9 +142,8 @@ def all_trainable_layers_are_registered(
     False otherwise.
   """
   for layer in input_model.layers:
-    for sublayer in _get_internal_layers(layer):
-      if not layer_registry.is_elem(sublayer) and sublayer.trainable_variables:
-        return False
+    if not layer_registry.is_elem(layer) and layer.trainable_variables:
+      return False
   return True
 
 
@@ -213,17 +191,53 @@ def add_aggregate_noise(
 
 def generate_model_outputs_using_core_keras_layers(
     input_model: tf.keras.Model,
+    custom_layer_set: Optional[Set[type]] = None,  # pylint: disable=g-bare-generic
 ) -> PackedTensors:
-  """Returns the model outputs generated by only core Keras layers."""
-  cust_obj_dict = dict.copy(tf.keras.utils.get_custom_objects())
-  cust_hash_set = set([hash(v) for v in cust_obj_dict.values()])
+  """Returns the model outputs generated by only core Keras layers.
+
+  Args:
+    input_model: A `tf.keras.Model` instance to obtain outputs from.
+    custom_layer_set: An optional `set` of custom layers to expand. If `None`,
+      then this is the set of all registered custom Keras layers.
+
+  Returns:
+    A `tf.Tensor` that is the result of `input_model(input_model.inputs)`
+    using only Keras layers that are not in `custom_layer_set`.
+  """
+  # Set up helper variables and functions.
+  custom_layer_set = (
+      custom_layer_set or tf.keras.utils.get_custom_objects().values()
+  )
+
+  def _is_core(layer_instance):
+    return type(layer_instance) not in custom_layer_set
 
   def generator_fn(layer_instance, args, kwargs):
-    if hash(layer_instance.__class__) in cust_hash_set:
-      # Using `.call()` does not register the layer in the compute graph of
-      # a forward pass.
-      return layer_instance.call(*args, **kwargs), None
-    else:
-      return layer_instance(*args, **kwargs), None
+    # Using `.call()` does not register the layer in the compute graph of
+    # a forward pass.
+    layer_outputs = (
+        layer_instance(*args, **kwargs)
+        if _is_core(layer_instance)
+        else layer_instance.call(*args, **kwargs)
+    )
+    return layer_outputs, None
 
-  return model_forward_pass(input_model, input_model.inputs, generator_fn)[0]
+  # Return early if all the existing layers contain only core layers.
+  if all(_is_core(layer) for layer in input_model.layers):
+    return model_forward_pass(input_model, input_model.inputs)[0]
+
+  # Do a forward pass to expand the outermost layers.
+  candidate_outputs, _ = model_forward_pass(
+      input_model, input_model.inputs, generator_fn
+  )
+
+  # The following recursion is inefficient because it recursively builds `n`
+  # Keras model graphs, where `n` is the number of recursive calls. However,
+  # it appears to be the only valid approach without accessing Keras's internal
+  # functions (e.g., `keras.engine.functional._map_graph_network()`).
+  cleaned_model = tf.keras.Model(
+      inputs=input_model.inputs, outputs=candidate_outputs
+  )
+  return generate_model_outputs_using_core_keras_layers(
+      cleaned_model, custom_layer_set
+  )

tensorflow_privacy/privacy/fast_gradient_clipping/gradient_clipping_utils_test.py

@@ -12,12 +12,72 @@
 # See the License for the specific language governing permissions and
 # limitations under the License.
 
+from typing import Any
+
 from absl.testing import parameterized
 import tensorflow as tf
 
 from tensorflow_privacy.privacy.fast_gradient_clipping import gradient_clipping_utils
 
 
+# ==============================================================================
+# Helper functions and classes.
+# ==============================================================================
+@tf.keras.utils.register_keras_serializable('gradient_clipping_utils_test')
+class DoubleDense(tf.keras.layers.Layer):
+  """Generates two dense layers nested together."""
+
+  def __init__(self, units: int):
+    super().__init__()
+    self.dense1 = tf.keras.layers.Dense(units, name='DDense_ext_1')
+    self.dense2 = tf.keras.layers.Dense(1, name='DDense_ext_2')
+
+  def call(self, inputs: Any):
+    x = self.dense1(inputs)
+    return self.dense2(x)
+
+
+@tf.keras.utils.register_keras_serializable('gradient_clipping_utils_test')
+class TripleDense(tf.keras.layers.Layer):
+  """Generates three dense layers nested together."""
+
+  def __init__(self, units: int):
+    super().__init__()
+    self.dense1 = tf.keras.layers.Dense(units, name='TDense_ext_1')
+    self.dense2 = tf.keras.layers.Dense(units, name='TDense_ext_2')
+    self.dense3 = tf.keras.layers.Dense(1, name='TDense_ext_3')
+
+  def call(self, inputs: Any):
+    x1 = self.dense1(inputs)
+    x2 = self.dense2(x1)
+    return self.dense3(x2)
+
+
+def get_reduced_model(sample_inputs, hidden_layer_list, new_custom_layers=None):
+  """Reduces a set of layers to only core Keras layers in a model."""
+  sample_outputs = sample_inputs
+  for l in hidden_layer_list:
+    sample_outputs = l(sample_outputs)
+  custom_model = tf.keras.Model(inputs=sample_inputs, outputs=sample_outputs)
+  if new_custom_layers:
+    reduced_outputs = (
+        gradient_clipping_utils.generate_model_outputs_using_core_keras_layers(
+            custom_model,
+            custom_layer_set=new_custom_layers,
+        )
+    )
+  else:
+    reduced_outputs = (
+        gradient_clipping_utils.generate_model_outputs_using_core_keras_layers(
+            custom_model
+        )
+    )
+  return tf.keras.Model(inputs=custom_model.inputs, outputs=reduced_outputs)
+
+
+# ==============================================================================
+# Main tests.
+# ==============================================================================
 class ModelForwardPassTest(tf.test.TestCase, parameterized.TestCase):
 
   @parameterized.product(
@@ -75,5 +135,46 @@ class ModelForwardPassTest(tf.test.TestCase, parameterized.TestCase):
     self.assertAllClose(computed_outputs, true_outputs)
 
 
+class GenerateOutputsUsingCoreKerasLayers(
+    tf.test.TestCase, parameterized.TestCase
+):
+
+  def test_single_custom_layer_is_reduced(self):
+    num_units = 5
+    num_dims = 3
+    reduced_model = get_reduced_model(
+        tf.keras.Input(num_dims),
+        [DoubleDense(num_units)],
+    )
+    # Ignore the input layer.
+    for l in reduced_model.layers[1:]:
+      self.assertIsInstance(l, tf.keras.layers.Dense)
+
+  def test_two_distinct_custom_layers_are_reduced(self):
+    num_units = 5
+    num_dims = 3
+    reduced_model = get_reduced_model(
+        tf.keras.Input(num_dims),
+        [DoubleDense(num_units), TripleDense(num_units)],
+    )
+    # Ignore the input layer.
+    for l in reduced_model.layers[1:]:
+      self.assertIsInstance(l, tf.keras.layers.Dense)
+
+  def test_new_custom_layer_spec(self):
+    num_units = 5
+    num_dims = 3
+    reduced_model = get_reduced_model(
+        tf.keras.Input(num_dims),
+        [DoubleDense(num_units), TripleDense(num_units)],
+        new_custom_layers=set([DoubleDense]),
+    )
+    # Ignore the input layer.
+    for l in reduced_model.layers[1:]:
+      self.assertTrue(
+          isinstance(l, tf.keras.layers.Dense) or isinstance(l, TripleDense)
+      )
+
+
 if __name__ == '__main__':
   tf.test.main()