SerializedShuffleHandle is a BaseShuffleHandle with no additional methods or fields and serves only to identify the choice of a serialized shuffle.
Like BaseShuffleHandle, SerializedShuffleHandle takes shuffleId, numMaps, and a ShuffleDependency.
SerializedShuffleHandle is created when SortShuffleManager is requested for a ShuffleHandle (for a ShuffleDependency) and the conditions hold (but for BypassMergeSortShuffleHandle do not which are checked first).