T5 fine-tuning `.generate()` issue 
Author: oozeren
Pull request: no

### System Info

```shell
i am trying to create own T5 fine-tuner but i get "TypeError: The current model class (ALGORITHMIAT5) is not compatible with `.generate()`, as it doesn't have a language model head."
```


### Information

- [ ] The official example scripts
- [ ] 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

class ALGORITHMIAT5(PreTrainedModel):

    config_class = ALGORITHMIAMODELConfig

    def __init__(self, config):
        super().__init__(config)
        self.config = config
        self.model =AutoModelForSeq2SeqLM.from_pretrained(**config, ignore_mismatched_sizes=True)
        self.tokenizer =   AutoTokenizer.from_pretrained(pretrained_model_name_or_path=pretrained_tokenizer_name_or_path,
                                              ignore_mismatched_sizes=True)

    def forward(self, input_ids, attention_mask=None, decoder_input_ids=None, decoder_attention_mask=None, labels=None):
        print(input_ids)
        return self.model(
            input_ids,
            attention_mask=attention_mask,
            decoder_input_ids=decoder_input_ids,
            decoder_attention_mask=decoder_attention_mask,
            labels=labels,
        )

### Expected behavior

```shell
train the model
```


### Checklist

- [X] I have read the migration guide in the readme. ([pytorch-transformers](https://github.com/huggingface/transformers#migrating-from-pytorch-transformers-to-transformers); [pytorch-pretrained-bert](https://github.com/huggingface/transformers#migrating-from-pytorch-pretrained-bert-to-transformers))
- [X] I checked if a related official extension example runs on my machine.

Comments:
This the error : "/opt/omniai/software/Miniconda/lib/python3.8/site-packages/transformers/generation/utils.py in _validate_model_class(self)
   1063             if generate_compatible_classes:
   1064                 exception_message += f" Please use one of the following classes instead: {generate_compatible_classes}"
-> 1065             raise TypeError(exception_message)
   1066 
   1067     def _validate_model_kwargs(self, model_kwargs: Dict[str, Any]):

TypeError: The current model class (ALGORITHMIAT5) is not compatible with `.generate()`, as it doesn't have a language model head.

The "google/flan-t5-small"  model is used.

Hey 🤗  thanks for opening an issue! We try to keep the github issues for bugs/feature requests.
Could you ask your question on the [forum](https://discuss.huggingface.co/) instead? I'm sure the community will be of help!

For you example, I would recommend you to properly define your class with
```python
def generate():
    self.model.generate()
```
or something like that! 

Thanks Arthur. Will do

On Wed, Feb 7, 2024 at 9:31 PM Arthur ***@***.***> wrote:

> Hey 🤗 thanks for opening an issue! We try to keep the github issues for
> bugs/feature requests.
> Could you ask your question on the forum <https://discuss.huggingface.co/>
> instead? I'm sure the community will be of help!
>
> For you example, I would recommend you to properly define your class with
>
> def generate():
>     self.model.generate()
>
> or something like that!
>
> —
> Reply to this email directly, view it on GitHub
> <https://github.com/huggingface/transformers/issues/28911#issuecomment-1933269590>,
> or unsubscribe
> <https://github.com/notifications/unsubscribe-auth/BF7NDUZQYEKMMNOLWYCSPGLYSQ2H7AVCNFSM6AAAAABC6GU3ZCVHI2DSMVQWIX3LMV43OSLTON2WKQ3PNVWWK3TUHMYTSMZTGI3DSNJZGA>
> .
> You are receiving this because you authored the thread.Message ID:
> ***@***.***>
>


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