Add dummy rule for keras_preprocessing so that it can be swapped by copybara in OSS.

PiperOrigin-RevId: 341971051
Change-Id: I1ae5bc36262499ea99716710eeeff194d5b427e8

Author: qlzh727

tensorflow/python/keras/BUILD

@@ -666,6 +666,14 @@ py_library(
     visibility = ["//visibility:public"],
 )
 
+py_library(
+    name = "expect_keras_preprocessing_installed",
+    # This is a dummy rule used as a keras_preprocessing dependency in open-source.
+    # We expect keras_preprocessing to already be installed on the system, e.g. via
+    # `pip install keras_preprocessing`
+    visibility = ["//visibility:public"],
+)
+
 py_library(
     name = "expect_numpy_installed",
     # This is a dummy rule used as a numpy dependency in open-source.