code: public ViewTransitionBuilder addViewTransformer(@NonNull ViewTransformer viewTransformer) {
          checkModifiability();
          if (mCustomTransitionController == null) {
              mCustomTransitionController = new CustomTransitionController();
          }
          mCustomTransitionController.addViewTransformer(viewTransformer);
          return self();
      }
docs: Adds a custom {@link ViewTransformer}, the builder must be cloned if this method  is called.
  
  @param viewTransformer
  @return