MT5Model(MT5Config()) fails with AttributeError: 'MT5Config' object has no attribute 'is_gated_act'
### System Info

```shell
current main branch, 4.20.0.dev0
```


### Who can help?

@patrickvonplaten @sgugger

### Information

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

### Tasks

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

### Reproduction

```
from transformers import *

model = MT5ForConditionalGeneration(MT5Config())
```

### Expected behavior

no error
I think it was introduced in https://github.com/huggingface/transformers/pull/17420<|||||>cc @patil-suraj as well.
It would also be interesting to understand how it was not caught by any of the tests.