createChildTransition

createChildTransition creates a child Transition based on the mapping between parent state to child state provided in transformToChildState. This serves the following purposes:

1. Hoist the child transition state into parent transition. Therefore the parent Transition will be aware of whether there's any on-going animation due to the same target state change. This will further allow sequential animation to be set up when all animations have finished.

2. Separation of concerns. The child transition can respresent a much more simplified state transition when, for example, mapping from an enum parent state to a Boolean visible state for passing further down the compose tree. The child composables hence can be designed around handling a more simple and a more relevant state change.

label is used to differentiate from other animations in the same transition in Android Studio.