AutoModelForSequenceClassification not working with prunebert model
I am having issues loading the new prunebert model for sequence classification using AutoModelForSequenceClassification.from_pretrained(). 

```
from transformers import AutoModelForSequenceClassification, BertForSequenceClassification
model = AutoModelForSequenceClassification.from_pretrained("huggingface/prunebert-base-uncased-6-finepruned-w-distil-mnli")
```
The above code produces the error: KeyError: 'masked_bert'.

The model loads fine using BertForSequenceClassification.from_pretrained, the issue only seems to occur with AutoModelForSequenceClassification. 

```
model = BertForSequenceClassification.from_pretrained('huggingface/prunebert-base-uncased-6-finepruned-w-distil-mnli')
```

This is because the `huggingface/prunebert-xxx` configurations have a model type `masked_bert`.

Since these files should be loaded directly in `BertForXXX` classes, it would probably be best to update that field to `bert`, right @julien-c?<|||||>Yes. Can you confirm @VictorSanh?<|||||>Seems to be working fine now, thanks @LysandreJik and @julien-c 