`tf.keras.mixed_precision.experimental.set_policy`
    *   In `Layer.call`, `AutoCastVariable`s will no longer be casted within
        `MirroredStrategy.run` or `ReplicaContext.merge_call`. This is because a
        thread local variable is used to determine whether `AutoCastVariable`s
        are casted, and those two functions run with a different thread. Note
        this only applies if one of these two functions is called within