Unable to load mT5 with FlaxAutoModelForSeq2SeqLM
## Environment info
- `transformers` version: 4.9.0.dev0
- Platform: TPU VM
- Python version: 3.8.10

### Who can help
@patrickvonplaten 

##
Trying to load mT5 ('google/mt5-small') with FlaxAutoModelForSeq2SeqLM leads to the following error:

```
>>> import transformers
>>> model = FlaxAutoModelForSeq2SeqLM.from_pretrained('google/mt5-small')

ValueError: Unrecognized configuration class <class 'transformers.models.mt5.configuration_mt5.MT5Config'> for this kind of AutoModel: FlaxAutoModelForSeq2SeqLM.
Model type should be one of BartConfig, T5Config.
```

Loading the same model with FlaxT5ForConditionalGeneration works fine. @patrickvonplaten suggested in Slack #flax-jax-community-week that the issue might be caused by missing MT5Config.