Add BUILD file for keras/initializers package.

The keras:initializers target and related tests has been moved to the new build file.

Also updating all the BUILD to pointing to the new location.

PiperOrigin-RevId: 375140497

Author: qlzh727

tensorflow_model_optimization/python/core/quantization/keras/layers/BUILD

@@ -28,7 +28,7 @@ py_strict_library(
         # python/framework:for_generated_wrappers tensorflow dep2,
         # python/keras:activations tensorflow dep2,
         # python/keras:backend tensorflow dep2,
-        # python/keras:initializers tensorflow dep2,
+        # python/keras/initializers tensorflow dep2,
         # python/keras/layers tensorflow dep2,
         # python/keras/layers:convolutional tensorflow dep2,
         # python/keras/layers/normalization:batch_normalization_v1 tensorflow dep2,