How to generate one token after the other when no past_key_values is returned?
### System Info

Python: 3.11
transformers==4.29.2
torch==2.0.1


### Who can help?

@ArthurZucker @younesbelkada @gante @sgugger

### 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

```
import torch
import torch.nn.functional as F
from transformers import AutoTokenizer, AutoModelForCausalLM

device = "cuda"
tokenizer = AutoTokenizer.from_pretrained('allenai/scibert_scivocab_uncased')
model = AutoModelForCausalLM.from_pretrained('allenai/scibert_scivocab_uncased').to(device)

input_sequence = "Hello, I'm a language model,"

inputs = torch.as_tensor(tokenizer.encode(input_sequence)).unsqueeze(0).to(device)
attention_mask = torch.as_tensor(tokenizer(input_sequence).attention_mask).unsqueeze(0).to(device)
past_key_values = None

count = 0
complete_token = []
with torch.no_grad():
    while count < 10:
        count += 1
        print("Iteration no.: " + str(count))
        if count > 1:
            inputs = input_token

        print(inputs.to(device))
        print(attention_mask)
        print(past_key_values[0][0].shape if past_key_values else None)

        model_out = model(input_ids=inputs.to(device), attention_mask=attention_mask, past_key_values=past_key_values)
        logits = model_out.logits[:, -1, :]
        past_key_values = model_out.past_key_values

        topk_values, topk_indices = torch.topk(logits, 5)

        log_probs = F.softmax(topk_values, dim=-1)
        inputs_in_topk = torch.multinomial(log_probs, num_samples=1, replacement=True)
        input_token = torch.gather(topk_indices, 1, inputs_in_topk)
        attention_mask = torch.concat((attention_mask, torch.tensor([[1]]).to(attention_mask.device)), dim=1)
        complete_token.append(input_token)
```

### Expected behavior

I would like to use Scibert for iterated token generation. Above is my code.

However, we have past_key_values = Null all the time. I tried this approach with other models and past_key_values is not null there. How can I make the iteration work here, such that we have the knowledge of the previous iteration?
This is an encoder model, so you can't use it as a decoder model for generation like this.<|||||>Hi @sgugger, many thanks for getting back to me. 

So is there no possibility to use that model for generative purposes?

Also, could we have some slight influence on the selection of each individual token?<|||||>Hi @junoriosity -- only model architectures with decoder blocks can be used for generative purposes, see [this page from our NLP course](https://huggingface.co/learn/nlp-course/chapter1/4?fw=pt#general-architecture). If these concepts are new to you, I'd recommend going through the course, which you can complete in a single day!

`allenai/scibert_scivocab_uncased`, a BERT model, so it doesn't have a decoder block :)<|||||>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/main/CONTRIBUTING.md) are likely to be ignored.