Add LongT5 to AutoConfig
### System Info

config = AutoConfig.from_pretrained("google/long-t5-local-base")

gives a KeyError: 'longt5'

I am trying to run this as it's part of SentenceTransformer package when initialising a search model - could longT5 be added to AutoConfig?



### Who can help?

_No response_

### Information

- [X] The official example scripts
- [ ] My own modified scripts

### Tasks

- [ ] An officially supported task in the `examples` folder (such as GLUE/SQuAD, ...)
- [X] My own task or dataset (give details below)

### Reproduction

config = AutoConfig.from_pretrained("google/long-t5-local-base")

### Expected behavior

The config is loaded
Hey @robbohua -- I can run the script you shared without problems on my end. What version of `transformers` are you using? :)<|||||>This issue has been automatically marked as stale because it has not had recent activity. If you think this still needs to be addressed please comment on this thread.

Please note that issues that do not follow the [contributing guidelines](https://github.com/huggingface/transformers/blob/main/CONTRIBUTING.md) are likely to be ignored.