Take two of keras-team#1812, simpler classifier head loading

Let's get rid of `load_task_extras`, which is a bad and confusing name.
Instead, we will adopt some behavior that is specific to classifiers,
but a lot simpler.

```python
# Random head.
classifier = ImageClassifier.from_preset("resnet50", num_classes=2)
# Pretrained head.
classifier = ImageClassifier.from_preset("resnet50")
# Error, must provide num_classes.
classifier = TextClassifier.from_preset("bert_base_en")
```

Author: mattdangerw

keras_nlp/src/models/preprocessor.py

@@ -126,7 +126,6 @@ def presets(cls):
     def from_preset(
         cls,
         preset,
-        load_task_extras=False,
         **kwargs,
     ):
         """Instantiate a `keras_nlp.models.Preprocessor` from a model preset.
@@ -150,9 +149,6 @@ def from_preset(
         Args:
             preset: string. A built-in preset identifier, a Kaggle Models
                 handle, a Hugging Face handle, or a path to a local directory.
-            load_task_extras: bool. If `True`, load the saved task preprocessing
-                configuration from a `preprocessing.json`. You might use this to
-                restore the sequence length a model was fine-tuned with.
 
         Examples:
         ```python
@@ -179,7 +175,7 @@ def from_preset(
         # Detect the correct subclass if we need to.
         if cls.backbone_cls != backbone_cls:
             cls = find_subclass(preset, cls, backbone_cls)
-        return loader.load_preprocessor(cls, load_task_extras, **kwargs)
+        return loader.load_preprocessor(cls, **kwargs)
 
     def save_to_preset(self, preset_dir):
         """Save preprocessor to a preset directory.

keras_nlp/src/models/preprocessor_test.py

@@ -121,7 +121,5 @@ def test_save_to_preset(self, cls, preset_name):
         self.assertEqual(set(tokenizer.file_assets), set(expected_assets))
 
         # Check restore.
-        restored = cls.from_preset(save_dir, load_task_extras=True)
+        restored = cls.from_preset(save_dir)
         self.assertEqual(preprocessor.get_config(), restored.get_config())
-        restored = cls.from_preset(save_dir, load_task_extras=False)
-        self.assertNotEqual(preprocessor.get_config(), restored.get_config())

keras_nlp/src/models/task.py

@@ -139,7 +139,6 @@ def from_preset(
         cls,
         preset,
         load_weights=True,
-        load_task_extras=False,
         **kwargs,
     ):
         """Instantiate a `keras_nlp.models.Task` from a model preset.
@@ -168,10 +167,6 @@ def from_preset(
             load_weights: bool. If `True`, saved weights will be loaded into
                 the model architecture. If `False`, all weights will be
                 randomly initialized.
-            load_task_extras: bool. If `True`, load the saved task configuration
-                from a `task.json` and any task specific weights from
-                `task.weights`. You might use this to load a classification
-                head for a model that has been saved with it.
 
         Examples:
         ```python
@@ -199,7 +194,12 @@ def from_preset(
         # Detect the correct subclass if we need to.
         if cls.backbone_cls != backbone_cls:
             cls = find_subclass(preset, cls, backbone_cls)
-        return loader.load_task(cls, load_weights, load_task_extras, **kwargs)
+        # Specifically for classifiers, we never load task weights if
+        # num_classes is supplied. We handle this in the task base class because
+        # it is the some logic for classifiers regardless of modality (text,
+        # images, audio).
+        load_task_weights = "num_classes" not in kwargs
+        return loader.load_task(cls, load_weights, load_task_weights, **kwargs)
 
     def load_task_weights(self, filepath):
         """Load only the tasks specific weights not in the backbone."""

keras_nlp/src/models/task_test.py

@@ -138,20 +138,16 @@ def test_save_to_preset(self):
         self.assertEqual(BertTextClassifier, check_config_class(task_config))
 
         # Try loading the model from preset directory.
-        restored_task = TextClassifier.from_preset(
-            save_dir, load_task_extras=True
-        )
+        restored_task = TextClassifier.from_preset(save_dir)
 
         # Check the model output.
         data = ["the quick brown fox.", "the slow brown fox."]
         ref_out = task.predict(data)
         new_out = restored_task.predict(data)
         self.assertAllClose(ref_out, new_out)
 
-        # Load without head weights.
-        restored_task = TextClassifier.from_preset(
-            save_dir, load_task_extras=False, num_classes=2
-        )
+        # Load without head different head weights.
+        restored_task = TextClassifier.from_preset(save_dir, num_classes=2)
         data = ["the quick brown fox.", "the slow brown fox."]
         # Full output unequal.
         ref_out = task.predict(data)

keras_nlp/src/models/text_classifier.py

@@ -32,6 +32,12 @@ class TextClassifier(Task):
     All `TextClassifier` tasks include a `from_preset()` constructor which can be
     used to load a pre-trained config and weights.
 
+    Some classification presets (but not all), include classification head
+    weights in a `task.weights.h5`. For these presets, you can omit passing
+    `num_classes` to re-create the save classification head. For all presets, if
+    `num_classes` is passed as a kwarg to `from_preset()`, the classification
+    head will be randomly initialized.
+
     Example:
     ```python
     # Load a BERT classifier with pre-trained weights.

keras_nlp/src/utils/preset_utils.py

@@ -673,7 +673,7 @@ def load_image_converter(self, cls, **kwargs):
         """Load an image converter layer from the preset."""
         raise NotImplementedError
 
-    def load_task(self, cls, load_weights, load_task_extras, **kwargs):
+    def load_task(self, cls, load_weights, load_task_weights, **kwargs):
         """Load a task model from the preset.
 
         By default, we create a task from a backbone and preprocessor with
@@ -689,11 +689,10 @@ def load_task(self, cls, load_weights, load_task_extras, **kwargs):
         if "preprocessor" not in kwargs:
             kwargs["preprocessor"] = self.load_preprocessor(
                 cls.preprocessor_cls,
-                load_task_extras=load_task_extras,
             )
         return cls(**kwargs)
 
-    def load_preprocessor(self, cls, load_task_extras, **kwargs):
+    def load_preprocessor(self, cls, **kwargs):
         """Load a prepocessor layer from the preset.
 
         By default, we create a preprocessor from a tokenizer with default
@@ -738,33 +737,25 @@ def load_image_converter(self, cls, **kwargs):
         converter_config = load_json(self.preset, IMAGE_CONVERTER_CONFIG_FILE)
         return load_serialized_object(converter_config, **kwargs)
 
-    def load_task(self, cls, load_weights, load_task_extras, **kwargs):
+    def load_task(self, cls, load_weights, load_task_weights, **kwargs):
         # If there is no `task.json` or it's for the wrong class delegate to the
         # super class loader.
-        if not load_task_extras:
-            return super().load_task(
-                cls, load_weights, load_task_extras, **kwargs
-            )
         if not check_file_exists(self.preset, TASK_CONFIG_FILE):
-            raise ValueError(
-                "Saved preset has no `task.json`, cannot load the task config "
-                "from a file. Call `from_preset()` with "
-                "`load_task_extras=False` to load the task from a backbone "
-                "with library defaults."
+            return super().load_task(
+                cls, load_weights, load_task_weights, **kwargs
             )
         task_config = load_json(self.preset, TASK_CONFIG_FILE)
         if not issubclass(check_config_class(task_config), cls):
-            raise ValueError(
-                f"Saved `task.json`does not match calling cls {cls}. Call "
-                "`from_preset()` with `load_task_extras=False` to load the "
-                "task from a backbone with library defaults."
+            return super().load_task(
+                cls, load_weights, load_task_weights, **kwargs
             )
         # We found a `task.json` with a complete config for our class.
         task = load_serialized_object(task_config, **kwargs)
         if task.preprocessor is not None:
             task.preprocessor.tokenizer.load_preset_assets(self.preset)
         if load_weights:
-            if check_file_exists(self.preset, TASK_WEIGHTS_FILE):
+            has_task_weights = check_file_exists(self.preset, TASK_WEIGHTS_FILE)
+            if has_task_weights and load_task_weights:
                 jax_memory_cleanup(task)
                 task_weights = get_file(self.preset, TASK_WEIGHTS_FILE)
                 task.load_task_weights(task_weights)
@@ -774,23 +765,14 @@ def load_task(self, cls, load_weights, load_task_extras, **kwargs):
             task.backbone.load_weights(backbone_weights)
         return task
 
-    def load_preprocessor(self, cls, load_task_extras, **kwargs):
-        if not load_task_extras:
-            return super().load_preprocessor(cls, load_task_extras, **kwargs)
+    def load_preprocessor(self, cls, **kwargs):
+        # If there is no `preprocessing.json` or it's for the wrong class,
+        # delegate to the super class loader.
         if not check_file_exists(self.preset, PREPROCESSOR_CONFIG_FILE):
-            raise ValueError(
-                "Saved preset has no `preprocessor.json`, cannot load the task "
-                "preprocessing config from a file. Call `from_preset()` with "
-                "`load_task_extras=False` to load the preprocessor with "
-                "library defaults."
-            )
+            return super().load_preprocessor(cls, **kwargs)
         preprocessor_json = load_json(self.preset, PREPROCESSOR_CONFIG_FILE)
         if not issubclass(check_config_class(preprocessor_json), cls):
-            raise ValueError(
-                f"Saved `preprocessor.json`does not match calling cls {cls}. "
-                "Call `from_preset()` with `load_task_extras=False` to "
-                "load the the preprocessor with library defaults."
-            )
+            return super().load_preprocessor(cls, **kwargs)
         # We found a `preprocessing.json` with a complete config for our class.
         preprocessor = load_serialized_object(preprocessor_json, **kwargs)
         preprocessor.tokenizer.load_preset_assets(self.preset)