BigBird
## Environment info
- `transformers` version: 4.4.0.dev0
- Platform: Linux-5.3.0-64-generic-x86_64-with-glibc2.30
- Python version: 3.9.2
- PyTorch version (GPU?): 1.8.0 (True)
- Tensorflow version (GPU?): not installed (NA)
- Using GPU in script?: Y
- Using distributed or parallel set-up in script?: N

### Who can help
@patrickvonplaten, @patil-suraj (guessing)
## Information

Model I am using **BigBird**:

The problem arises when using: the official example scripts: **seq2seq**

The tasks I am working on is: the official **BigPatent** dataset

## To reproduce
I'm trying to use BigBird for a summarization task (on the BigPatent dataset). I'm using the official seq2seq script, which I run as (all lengths/batches are small for testing)

```
python run_seq2seq.py \
    --model_name_or_path google/bigbird-roberta-base \
    --dataset_name big_patent \
    --max_source_length 3 \
    --max_target_length 3 \
    --val_max_target_length 3 \
    --do_eval --do_predict \
    --per_gpu_train_batch_size 1 \
    --per_gpu_eval_batch_size 1 \
    --num_train_epochs 1 \
    --output_dir tmp 
```

However, I get the following error message:
```
Traceback (most recent call last):
  File "/home/scasola/factuality/factuality/transformers/examples/seq2seq/run_seq2seq.py", line 657, in <module>     
    main()
  File "/home/scasola/factuality/factuality/transformers/examples/seq2seq/run_seq2seq.py", line 344, in main
    config = AutoConfig.from_pretrained(
  File "/home/scasola/anaconda3/envs/factuality/lib/python3.9/site-packages/transformers/models/auto/configuration_auto.py", line 382, in from_pretrained
    config_class = CONFIG_MAPPING[config_dict["model_type"]]
KeyError: 'big_bird'
```
hi @slvcsl 

`BigBird` is till WIP and not yet added into the lib,. You can follow the progress here #10183

<|||||>Thank you for your response. I'll check it out!<|||||>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/master/CONTRIBUTING.md) are likely to be ignored.