Relax `eos_token_id < 0` checks in `generate()` from `ValueError` to warning
Author: lewtun
Pull request: yes

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This PR relaxes a constraint that was introduced in eec46b4 to ensure `eos_token_id > 0` when `min_new_tokens > 0`. In particular, the current code would throw a `ValueError`:

```python
from transformers import pipeline

pipe = pipeline("text-generation")
pipe("Hello, my dog is cute.", min_new_tokens=4, eos_token_id=-1)
```

This isn't ideal as setting `eos_token_id=-1` is a handy trick to enable generating text past the EOS token. The current PR now returns a warning instead. I've also taken the liberty to relax the error on `min_length < 0` but let me know if that's not a good idea.

Internal Slack thread where this was discussed: https://huggingface.slack.com/archives/C01N44FJDHT/p1680175455762649


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Comments:
_The documentation is not available anymore as the PR was closed or merged._

Thanks for the review @sgugger - I've now fixed the logging and reverted the change on `min_length`