UL2: UL2 is a T5 like model pretrained on various denoising objectives

Flan-T5: Flan is a pretraining methods that is based on prompting. The Flan-T5 are T5 models trained on the Flan collection of 
    datasets which include: taskmaster2, djaym7/wiki_dialog, deepmind/code_contests, lambada, gsm8k, aqua_rat, esnli, quasc and qed.

FLan-UL2 : the UL2 model finetuned using the "Flan" prompt tuning and dataset collection.