Add UL2: Unifying Language Learning Paradigms
### Model description

UL2 is a unified framework for pretraining models that are universally effective across datasets and setups. UL2 uses Mixture-of-Denoisers (MoD), a pre-training objective that combines diverse pre-training paradigms together. UL2 introduces a notion of mode switching, wherein downstream fine-tuning is associated with specific pre-training schemes.

### Open source status

- [X] The model implementation is available
- [X] The model weights are available

### Provide useful links for the implementation

Code and weights (20 billion parameter models): https://github.com/google-research/google-research/tree/master/ul2
The code is based on T5x (which is JAX/FLAX): https://github.com/google-research/t5x
cc @stefan-it @peregilk @agemagician @stancld @edugp   FYI might be interesting for you as well :-) <|||||>Is anyone working on porting UL2 to `transformers` already? If not, I am interested in porting it.<|||||>Hey @manuelciosici - think @kamalkraj is working on it . Maybe you guys can sync on how to collaborate? :-) 

Happy to help in any way! <|||||>Hi @manuelciosici,

I am trying to understand the [t5x](https://github.com/google-research/t5x) library and loading the model.

We can work together. You can ping me on slack/discord
<|||||>@manuelciosici and @kamalkraj. I am about to start some UL2 training in t5x. I might also contribute here. <|||||>Hello @kamalkraj, regarding the `t5x` library (loading model, etc.), I've done some inference with `LongT5` model in my repo [here](https://github.com/stancld/longt5-eval).<|||||>Thank you so much @stancld 
<|||||>> @manuelciosici and @kamalkraj. I am about to start some UL2 training in t5x. I might also contribute here.

Hi @manuelciosici ,
Did you start fine-tuning? Did you identify the t5 gin file required for it.

They have only released `ul2` gin file. Not the full set. 
https://github.com/google-research/google-research/issues/1101
<|||||>@kamalkraj Unfortunately, I was handed a tight deadline, so I won't be able to look into UL2 until July.<|||||>no worries! Anybody interested in taking over the UL2 implementation ? Would be happy to help :-)<|||||>I can take a stab at this in the next week if no one else is actively working on it!

I'll hopefully open a PR soon - help is welcome from anyone who would like as well :)<|||||>I've had the model running locally for a while but didn't get around to pushing it to the hub until now 😅 

with #17420, merged into master the architecture is already supported (in 4.20).

I've put the weights here for now:
https://huggingface.co/Seledorn/ul2

I think what remains is mostly verifying that we get identical output with the port and the original model. But this is as @kamalkraj noted a bit difficult without the complete gin files.  Though the model does give me reasonable outputs, so I believe the conversion is at least mostly correct.
<|||||>That's amazing @DanielHesslow - I'll check them out this week!<|||||>Great job on porting the model!<|||||>Google released weights for 3 UL2 checkpoints. I'm assuming the model in HuggingFace corresponds to the last checkpoint, but just to make sure, that is correct right?<|||||>Yes that's true as I know! cc @DanielHesslow just to be sure as he has ported the checkpoint :-)<|||||>Yeah it's the latest one<|||||>Hello :hand:  are you aware of any implementation of the Mixture-of-Denoisers loss? preferably with HF compatibility. Thanks in any case!<|||||>We haven't added this one yet - would you like to open a feature request / PR for it maybe? :-) 