Remove private Keras imports.

PiperOrigin-RevId: 564240456

Author: fchollet

tensorflow_model_optimization/python/core/quantization/keras/experimental/default_n_bit/default_n_bit_transforms.py

@@ -28,12 +28,15 @@
 from tensorflow_model_optimization.python.core.quantization.keras.experimental.default_n_bit import default_n_bit_quantize_registry
 from tensorflow_model_optimization.python.core.quantization.keras.graph_transformations import transforms
 
-
 try:
-  from keras.backend import unique_object_name  # pylint: disable=g-import-not-at-top
+  import keras  # pylint: disable=g-import-not-at-top
+  if hasattr(keras, 'src'):
+    # Path as seen in pip packages as of TF/Keras 2.13.
+    from keras.src.backend import unique_object_name  # pylint: disable=g-import-not-at-top,g-importing-member
+  else:
+    from keras.backend import unique_object_name  # pylint: disable=g-import-not-at-top,g-importing-member
 except ImportError:
-  # Path as seen in pip packages as of TF/Keras 2.13.
-  from keras.src.backend import unique_object_name  # pylint: disable=g-import-not-at-top
+  unique_object_name = tf._keras_internal.backend.unique_object_name  # pylint: disable=protected-access
 
 LayerNode = transforms.LayerNode
 LayerPattern = transforms.LayerPattern