LongT5 Summarization Example Not Working
### System Info

- OS: Ubuntu 20.04.4 LTS focal
- Conda: 4.12.0
- Python: 3.7.14
- Pip: 22.1.2
- Torch: 1.12.0
- Transfromers: 4.16.2
- NVIDIA-SMI: 510.73.05
- Nvcc -V: cuda V11.3.109

### Who can help?

@patrickvonplaten, @ydshieh, @sgugger 

### Information

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

### Tasks

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

### Reproduction

`git clone --branch v4.16.2-release https://github.com/huggingface/transformers`

Example from [Transformers/examples/pytorch/summarization](https://github.com/huggingface/transformers/tree/main/examples/pytorch/summarization), the only change is the `--model_name_or_path`
```
python transformers/examples/pytorch/summarization/run_summarization.py \
    --model_name_or_path google/long-t5-tglobal-base \
    --do_train \
    --do_eval \
    --dataset_name cnn_dailymail \
    --dataset_config "3.0.0" \
    --source_prefix "summarize: " \
    --output_dir /tmp/tst-summarization \
    --per_device_train_batch_size=4 \
    --per_device_eval_batch_size=4 \
    --overwrite_output_dir \
    --predict_with_generate
```
**Error:**
```
[INFO|configuration_utils.py:644] 2022-07-16 13:23:14,077 >> loading configuration file https://huggingface.co/google/long-t5-tglobal-base/resolve/main/config.json from cache at /home/good/.cache/huggingface/transformers/1b9067139467923bb0ea7749ceb5694acb0950b479ad1ebe47d9014180af8c31.69c5bfb92a1a084ead5ef0d9c9c9f09bac4f07cfd875433aa8fab59199208a7f
Traceback (most recent call last):
  File "transformers/examples/pytorch/summarization/run_summarization.py", line 698, in <module>
    main()
  File "transformers/examples/pytorch/summarization/run_summarization.py", line 371, in main
    use_auth_token=True if model_args.use_auth_token else None,
  File "/home/good/anaconda3/envs/gpt1/lib/python3.7/site-packages/transformers/models/auto/configuration_auto.py", line 632, in from_pretrained
    config_class = CONFIG_MAPPING[config_dict["model_type"]]
  File "/home/good/anaconda3/envs/gpt1/lib/python3.7/site-packages/transformers/models/auto/configuration_auto.py", line 347, in __getitem__
    raise KeyError(key)
KeyError: 'longt5'
```

### Expected behavior

LongT5 model from [google/long-t5-tglobal-base](https://huggingface.co/google/long-t5-tglobal-base) to start training like a normal T5 model ([T5-base](https://huggingface.co/t5-base)).
update transformer to 4.20.0+ should solve this issue.

Also I don't think you need --source_prefix "summarize: "  according to the paper.<|||||>@whaleloops It works, thanks.