Cherrypick for RandomContrast update (#16435)

* Enable the keras dtensor API in OSS.

PiperOrigin-RevId: 438858608

* Switching learning/brain dependency to OSS compatible test_util

This is one test file failing, due to the monkey patching happens in the dtensor.init(), and I will need to dig more about the root cause (probably due to patching tf.Variable with DVariable, and cause logic difference for instance type checking.)

PiperOrigin-RevId: 439676157

* Update the docstring for keras.dtensor components.

1. Add docstring for LayoutMap.
2. Hide certain methods for keras.dtensor.optimizers.

PiperOrigin-RevId: 442651549

* add no-op label augmentation to RandomContrast

PiperOrigin-RevId: 439938098

Co-authored-by: Luke Wood <lukewood@google.com>

Author: qlzh727

keras/layers/preprocessing/image_preprocessing.py

@@ -1438,6 +1438,9 @@ def augment_image(self, image, transformation=None):
     output.set_shape(image.shape)
     return output
 
+  def augment_label(self, label, transformation=None):
+    return label
+
   def compute_output_shape(self, input_shape):
     return input_shape