Add UL2 (just docs)
Author: patrickvonplaten
Pull request: yes

# What does this PR do?

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Adds docs for UL2: https://huggingface.co/google/ul2 -> important model that deserves its own doc page IMO


## Before submitting
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Models:

- albert, bert, xlm: @LysandreJik
- blenderbot, bart, marian, pegasus, encoderdecoder,  t5: @patrickvonplaten, @patil-suraj
- longformer, reformer, transfoxl, xlnet: @patrickvonplaten
- fsmt: @stas00
- funnel: @sgugger
- gpt2: @patrickvonplaten, @LysandreJik
- rag: @patrickvonplaten, @lhoestq
- tensorflow: @LysandreJik

Library:

- benchmarks: @patrickvonplaten
- deepspeed: @stas00
- ray/raytune: @richardliaw, @amogkam
- text generation: @patrickvonplaten
- tokenizers: @n1t0, @LysandreJik
- trainer: @sgugger
- pipelines: @LysandreJik

Documentation: @sgugger

HF projects:

- datasets: [different repo](https://github.com/huggingface/datasets)
- rust tokenizers: [different repo](https://github.com/huggingface/tokenizers)

Examples:

- maintained examples (not research project or legacy): @sgugger, @patil-suraj
- research_projects/bert-loses-patience: @JetRunner
- research_projects/distillation: @VictorSanh

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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Shouldn't we also add a conversion script? Or isn't this required?

> Shouldn't we also add a conversion script? Or isn't this required?

It should be the same as: https://github.com/huggingface/transformers/blob/main/src/transformers/models/t5/convert_t5x_checkpoint_to_flax.py 

cc @DanielHesslow 

Yeah I based the conversion off of that script and mostly added a bunch of hacks to work around limitations of my local system. At some point there should probably be a stable conversion script from t5x, but for now the script above is good enough.

> Thanks for adding this! I'm curious why the empty module? Does one of the script complain if we don't add it?

Copied it more or less from dialogpt after I noticed one check repo test was failing. Re-iterated and it looks like the only thing that is required is that the name is in `configuration_auto.py` - thanks for double-checking here!

> Thanks for adding this! I'm curious why the empty module? Does one of the script complain if we don't add it?

Copied it more or less from dialogpt after I noticed one check repo test was failing. Re-iterated and it looks like the only thing that is required is that the name is in `configuration_auto.py` - thanks for double-checking here!