Support freeze parameters

This adds a method to support freezing and unfreezing a parameter for transfer
learning. There is also a helper to (un)freeze all paremters in a block, but
without filtering. For more advanced use cases of (un)freezing part of the
parameters in a block, it should be implemented using block.getChildren(),
block.getDirectParameters() and/or block.getParameters().

api/src/main/java/ai/djl/nn/Block.java

@@ -268,6 +268,17 @@ default NDList forward(
     void loadParameters(NDManager manager, DataInputStream is)
             throws IOException, MalformedModelException;
 
+    /**
+     * Freezes or unfreezes all parameters inside the block for training.
+     *
+     * @see Parameter#freeze(Boolean)
+     */
+    default void freezeParameters(Boolean freeze) {
+        for (Parameter parameter : getParameters().values()) {
+            parameter.freeze(freeze);
+        }
+    }
+
     /**
      * Validates that actual layout matches the expected layout.
      *

api/src/main/java/ai/djl/nn/Parameter.java

@@ -133,6 +133,22 @@ public boolean requiresGradient() {
         return requiresGrad;
     }
 
+    /**
+     * Freezes or unfreezes the parameter for training.
+     *
+     * <p>Sometimes during training, especially during transfer learning, it is typical to train
+     * only part of the model. For this, the freeze can be used to prevent certain parts from being
+     * trained.
+     *
+     * <p>This modifies the {@link #requiresGradient()} of the parameter.
+     *
+     * @param freeze whether the parameter should be frozen ({@code freeze == !requiresGradient()})
+     */
+    public void freeze(Boolean freeze) {
+        requiresGrad = !freeze;
+        array.setRequiresGradient(requiresGrad);
+    }
+
     /**
      * Checks if this {@code Parameter} is initialized.
      *

integration/src/main/java/ai/djl/integration/tests/training/GradientCollectorIntegrationTest.java

@@ -13,13 +13,17 @@
 package ai.djl.integration.tests.training;
 
 import ai.djl.Model;
+import ai.djl.basicmodelzoo.basic.Mlp;
 import ai.djl.ndarray.NDArray;
 import ai.djl.ndarray.NDArrays;
+import ai.djl.ndarray.NDList;
 import ai.djl.ndarray.NDManager;
 import ai.djl.ndarray.types.DataType;
 import ai.djl.ndarray.types.Shape;
+import ai.djl.nn.Block;
 import ai.djl.nn.Blocks;
 import ai.djl.nn.Parameter;
+import ai.djl.nn.SequentialBlock;
 import ai.djl.nn.core.Linear;
 import ai.djl.testing.Assertions;
 import ai.djl.testing.TestRequirements;
@@ -73,6 +77,61 @@ public void testAutograd() {
         }
     }
 
+    @Test
+    public void testFreezeParameters() {
+        try (Model model = Model.newInstance("model")) {
+            Block blockFrozen = new Mlp(10, 10, new int[] {10});
+            Block blockNormal = new Mlp(10, 10, new int[] {10});
+            Block combined = new SequentialBlock().add(blockFrozen).add(blockNormal);
+            model.setBlock(combined);
+
+            TrainingConfig config =
+                    new DefaultTrainingConfig(Loss.l2Loss())
+                            .optInitializer(Initializer.ONES, Parameter.Type.WEIGHT);
+
+            try (Trainer trainer = model.newTrainer(config)) {
+                trainer.initialize(new Shape(1, 10));
+
+                blockFrozen.freezeParameters(true);
+
+                // Find total params
+                Float frozenVal =
+                        blockFrozen.getParameters().valueAt(0).getArray().sum().getFloat();
+                Float normalVal =
+                        blockNormal.getParameters().valueAt(0).getArray().sum().getFloat();
+
+                // Run training step
+                NDManager manager = trainer.getManager();
+                NDArray data = manager.arange(100.0f).reshape(new Shape(10, 10));
+                NDArray labels = manager.arange(100.0f).reshape(new Shape(10, 10));
+                Batch batch =
+                        new Batch(
+                                manager, new NDList(data), new NDList(labels), 1, null, null, 0, 1);
+                EasyTrain.trainBatch(trainer, batch);
+                trainer.step();
+
+                // Check updated total params
+                // The frozen one should not have changed, but normal one should
+                Float newFrozenVal =
+                        blockFrozen.getParameters().valueAt(0).getArray().sum().getFloat();
+                Float newNormalVal =
+                        blockNormal.getParameters().valueAt(0).getArray().sum().getFloat();
+                Assert.assertEquals(newFrozenVal, frozenVal);
+                Assert.assertNotEquals(newNormalVal, normalVal);
+
+                blockFrozen.freezeParameters(false);
+
+                // Check that unfreezing the block now makes it update
+                EasyTrain.trainBatch(trainer, batch);
+                trainer.step();
+
+                Float nowUnfrozenVal =
+                        blockFrozen.getParameters().valueAt(0).getArray().sum().getFloat();
+                Assert.assertNotEquals(nowUnfrozenVal, frozenVal);
+            }
+        }
+    }
+
     @Test
     public void testTrain() throws IOException, TranslateException {
         TestRequirements.nightly();