Fix some Ruff rule B008 violations.

For remaining B008 violations, see [Issue 6945](tensorflow#6945)

tfx/examples/bert/utils/bert_models.py

@@ -13,6 +13,7 @@
 # limitations under the License.
 """Configurable fine-tuning BERT models for various tasks."""
 
+from __future__ import annotations
 from typing import Optional, List, Union
 
 import tensorflow as tf
@@ -59,8 +60,7 @@ def build_bert_classifier(bert_layer: tf.keras.layers.Layer,
 
 def compile_bert_classifier(
     model: tf.keras.Model,
-    loss: tf.keras.losses.Loss = tf.keras.losses.SparseCategoricalCrossentropy(
-        from_logits=True),
+    loss: tf.keras.losses.Loss | None = None,
     learning_rate: float = 2e-5,
     metrics: Optional[List[Union[str, tf.keras.metrics.Metric]]] = None):
   """Compile the BERT classifier using suggested parameters.
@@ -79,6 +79,9 @@ def compile_bert_classifier(
   Returns:
     None.
   """
+  if loss is None:
+    loss = tf.keras.losses.SparseCategoricalCrossentropy(from_logits=True)
+
   if metrics is None:
     metrics = ["sparse_categorical_accuracy"]