Export Generated Text 1 Token at a Time
### Feature request

When using the text-generation pipeline. We would like to be able export each token as it is generated. Currently we have to wait for the generation to be completed to view the results. 

### Motivation

Using text-generation in a production environment, this would greatly improve the user experience. Users currently have to wait for text to be generated. If we are able to implement this they could read text as it is generated by the models. 

### Your contribution

I would be able to bug check this feature if it was added!
WDYT of such a feature @Narsil?<|||||>I like the idea.

How would that look like though code wise ?

```python

pipe = pipeline('text-generation")
# Regular usage

generated = pipe("This is my prompt")

for out in pipe("This is my prompt", continuous=True):
    # out = [{"generated_text": " and"}]
    #
```
The biggest caveat with this idea is this parameters will probably be hard to cumulate with things like `batch_size` and `num_beams`. We can disable some options if some combinations of arguments are provided, but in general I prefer when all combinations of parameters are available.

Other idea would be to somehow add a callback within `generate` to receive the ids also as they come in.
What I don't like, is callback (not easy to work with and debug), but it could be much easier to implement, since we're just injecting something within `generate`.

```python
def intermediate_results(out):
    print(out)
    
pipe = pipeline("text-generation")
out = pipe("This is my prompt", continous_fn=print_intermediate_results))
```

Pinging @patrickvonplaten to see if you have ideas to get continuous tokens within `generate`.<|||||>@gante @patil-suraj could you take a look here?<|||||>As @Narsil said, in greedy search/sample generation, we can loop over and call generation with one new token at a time. The performance penalty is not that big, a bit over 2x ([on colab](https://colab.research.google.com/drive/1BQgO3HBRs7sYpKCGFs4LpXvmZFJ0QWEp?usp=sharing), the penalty probably grows with sequence length), and is trivial to implement.

For beam search generation, either there is some channel to push sequences as they are generated, or the whole generation logic is somehow exposed to correctly keep track of running sequences/scores. The latter seems unfeasible, the former could be done e.g. with some form of asynchronous queue (one thread runs generate and pushes to the queue, another reads from the queue). 

I'm not experienced in these matters, but... the cost/benefit ratio doesn't seem good (for beam search) 😅 <|||||>I like the idea, but I think it won't be trivial to implement given the current complexity of `generate`. Even for greedy search/sampling, simply calling `generate` for one token at a time will be very slow, as it won't be able to take advantage of caching.

Adding callback seems a good idea IMO as it won't clutter `generate` a lot. wdyt @patrickvonplaten @gante <|||||>Both can leverage the current `generate` and do NOT call `generate` 1 step at a time in my mind.
Both would use a callback within `generate` but the idea is to understand how a user would use those results.

I was merely asking how it should look live from a pipeline user perspective.<|||||>As a user OpenAI deal with this quite well. 

They use server sent events to send over partial completions - aka the JavaScript EventSource library

See “stream”
https://beta.openai.com/docs/api-reference/completions/create<|||||>To be honest, I'm not in favor of adding this to `generate` - it's too much of a nice-to-have feature and would unnecessarily increase maintenance and make `generate` much harder to understand than it already is<|||||>If it's possible to make it easy and clean with a general `callbacks: Optional[GenerationCallback] = None` function arg I think I'd be fine with it though, but would need to see a PR for it<|||||>Then inside `generate()` ideally we only have one `if callbacks is not None: then call all callbacks` code<|||||>```python
from transformers import pipeline
import torch
import threading
from transformers.generation_stopping_criteria import StoppingCriteria, StoppingCriteriaList
from queue import Queue


pipe = pipeline(model="hf-internal-testing/tiny-random-gpt2", task="text-generation", device=0)


class Stream(StoppingCriteria):
    def __init__(self, q):
        self.q = q

    def __call__(self, input_ids, scores) -> bool:
        self.q.put(input_ids)
        return False


queue = Queue()


def gen():
    pipe.model.generate(
        torch.LongTensor([[0, 1, 2]]).cuda(),
        stopping_criteria=StoppingCriteriaList([Stream(queue)]),
        max_new_tokens=10,
    )
    print("Finished generation")
    queue.put(False)


threading.Thread(target=gen).start()

while True:
    i = queue.get()
    if i is False:
        break
    else:
        print("Got i", pipe.tokenizer.decode(i[0]))
```

What do you think about this ?

I thought this would be an elegant solution to the problem.
Basically send generate to another thread and wait for results as they are coming.

The main drawback for pipelines as I said, is the other parameters combinations + backward compatibility support.  (+ Threads are a nightmare and if users are already using pipelines within thread/async/multiprocessing bad things might happen)<|||||>I'd be fine with this design - think it's nice! Think we should maybe put it under a new class though, called `Callback` instead of `StoppingCriteria` ?<|||||>> Think we should maybe put it under a new class though, called Callback instead of StoppingCriteria ?

Yes for sure, this was the minimal code, definitely not fit for merge.
Again, lots of caveats too with this approach, but at least it could be implemented relatively fast.<|||||>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.<|||||>Has there been any progress on this since last year?

I am interested in generating one token at a time for an interactive text generation web UI. But simply calling `model.generate` with `max_new_tokens=1` multiple times is a lot slower (about 2x) than generating all the tokens at once.<|||||>@oobabooga no progress, but I have it in my backlog for exploration. Very unlikely that it will see the light of day in the next ~6 months, though :)<|||||>FYI, I made a streaming generation service for Hugging Face [transformers](https://github.com/huggingface/transformers) that is fully compatible with the OpenAI API: https://github.com/hyperonym/basaran