Make the base task as metaclass and decorate methods not implemented.

PiperOrigin-RevId: 316712226

official/core/base_task.py

@@ -14,15 +14,18 @@
 # limitations under the License.
 # ==============================================================================
 """Defines the base task abstraction."""
+import abc
 import functools
 from typing import Any, Callable, Optional
 
+import six
 import tensorflow as tf
 
 from official.modeling.hyperparams import config_definitions as cfg
 from official.utils import registry
 
 
+@six.add_metaclass(abc.ABCMeta)
 class Task(tf.Module):
   """A single-replica view of training procedure.
 
@@ -54,14 +57,13 @@ class Task(tf.Module):
     """
     pass
 
+  @abc.abstractmethod
   def build_model(self) -> tf.keras.Model:
     """Creates the model architecture.
 
     Returns:
       A model instance.
     """
-    # TODO(hongkuny): the base task should call network factory.
-    pass
 
   def compile_model(self,
                     model: tf.keras.Model,
@@ -98,6 +100,7 @@ class Task(tf.Module):
       model.test_step = functools.partial(validation_step, model=model)
     return model
 
+  @abc.abstractmethod
   def build_inputs(self,
                    params: cfg.DataConfig,
                    input_context: Optional[tf.distribute.InputContext] = None):
@@ -112,7 +115,6 @@ class Task(tf.Module):
     Returns:
       A nested structure of per-replica input functions.
     """
-    pass
 
   def build_losses(self, labels, model_outputs, aux_losses=None) -> tf.Tensor:
     """Standard interface to compute losses.