code: public ViewTransitionBuilder transitViewGroup(@NonNull ViewGroupTransition viewGroupTransition) {
          ViewGroup vg = (ViewGroup) mView;
          int total = vg.getChildCount();
          View view;
          ViewGroupTransitionConfig config = new ViewGroupTransitionConfig(vg, total);
          for (int i = 0; i < total; i++) {
              view = vg.getChildAt(i);
              config.childView = view;
              config.index = i;
              viewGroupTransition.transit(target(view), config);
          }
          return self();
      }
docs: The view previously set (through {@link #target(View)}) is casted as a ViewGroup, and the specified
  {@link ViewGroupTransition} will {@link ViewGroupTransition#transit(ViewTransitionBuilder, ViewGroupTransitionConfig)}
  all the children views.
  
  @param viewGroupTransition
  @return
  @throws ClassCastException If the target view is not a ViewGroup.