MT5ForConditionalGeneration has model.config.max_length=20 by default. Why?
- `transformers` version: 4.6.1
- Platform: Ubuntu 18
- Python version: 3.6

I spent one week training a T5 model with this package and couldn't figure out why my sequences obtained with Trainer.evaluate were only yielding a maximum of 20 tokens. I sent the `max_length` argument to the tokenizer to encode the input/output.
After a long time I found out that this happens:
```
model = MT5ForConditionalGeneration.from_pretrained('google/mt5-small')
model.config.max_length
Out: 20
```
The generate method was being used in Trainer because I used `predict_with_generate=True`. 
**Please change this behaviour, this was a very hard bug to find.  `model.config.max_length` should be set to `None` by default, if the model does not have limitations.**
Hi @JoaoLages 👋  Tagging @patrickvonplaten for the discussion.

Context for this discussion: We have had similar problems in the past, related to a small default `max_length` (e.g. https://github.com/huggingface/transformers/issues/16622). We are now setting proper checks for the `min_length` arguments (it must be smaller than `max_length`, https://github.com/huggingface/transformers/pull/16668).

The issue is a double-edged sword. With no default, we risk generating beyond the model's input size, which may be undesirable. If we set `max_length` to the model's input size by default, we avoid the previous problem but `generate()` will still likely need significant resources (compute and memory), which may be frustrating to new users. With a small default, we run into the problem you described.

I agree that the current state is not good enough, but increasing the default `max_length` may open a pandora's box of problems. Would it help if the console showed why generation stopped? (which is one of the following: max length reached, all sentences finished and early stopping was active, or no further score improvement was possible)

Alternatively, we can increase the default to the model's input size, and rely on early stopping to ensure generation does not drag for long by default -- WDYT @patrickvonplaten? (we could add a tqdm-like progress bar so users don't think the process died)<|||||>> Would it help if the console showed why generation stopped?
Would still be hard to find why `max_length` was reached. 

I still think it's best to always have `max_length` defined by default to the maximum value that the model can handle. A warning could be shown to know that this behaviour is in usage and that it may consume a lot of resources.<|||||>Sadly we cannot change this default anymore due to backward compatibility. Always having the model generate up to maximum allowed tokens can also be tricky - multiple models will always error out due to memory, some models like T5 have no max length really, ... so think we'll have to leave it at 20. Maybe we can improve the docs somehow<|||||>I have the idea that no one is using early stopping in these models because 'it doesn't work' and it is due to this silent behaviour 😅 <|||||>These legacy choices are painful, but changing them also causes many issues for downstream users 💔 

@JoaoLages I've added three actions points for me:
1. Keep an eye on related issues, and bring up the discussion around this argument's default if this becomes a routine problem;
2. Try to log the stopping criteria (as discussed above), which may help to raise awareness around this argument;
3. Work on documentation, which I was going to anyway :D 

Is there anything else you believe we can do to improve `generate()`?<|||||>Not really, `generate` works perfectly, it was just this silent small max_length setting :)
Some warnings/log messages would be better than nothing!<|||||>(going to keep the issue open to backlink in a future improved logging PR)<|||||>People that are familiar with `generate()` should know that `max_length` can and should be overwritten. I'll try to make the docs better here, but I don't think we should add a warning as this will literally be shown everytime someone calls generate without defining `max_length`<|||||>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.