*   Updated `tf.keras.layers.BatchNormalization` to support masking of the inputs (`mask` argument) when computing the mean and variance.
 *   Add `tf.keras.layers.Identity`, a placeholder pass-through layer.
 *   Add `show_trainable` option to `tf.keras.utils.model_to_dot` to display layer trainable status in model plots.

#     rbe_linux: General RBE options used on all linux builds.
#     rbe_win_base:   General RBE options used on all Windows builds. Not to be used standalone.
#     rbe_win_clang:  Options specific to compiling using Clang.
#
#     rbe_linux_cpu:                  RBE options to build with only CPU support.
#     rbe_linux_cuda:                 RBE options to build with GPU support using clang.