Enabling users to provide their own `stopping_criteria` + `logits_processor` to `generate`.
Author: Narsil
Pull request: yes

# What does this PR do?

Fixes #12118

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## Before submitting
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Comments:
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.

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@patrickvonplaten  (Not urgent, get some rest :))

Sorry for the late reply here @Narsil - I'm happy with the PR I think :-) If we could add a test that would be great

@patrickvonplaten Should I merge this ?

I think we shouldn't check anything. If you defined something we pass it `as-is` IMO. It's a poweuser feature, the doc specifically mentions this:

https://github.com/huggingface/transformers/pull/12219/files#diff-b7601d397d5d60326ce61a9c91beaa2afa026014141052b32b07e1d044fbbe17R801

But also happy to drop the PR, the issue didn't seem to generate that much traction.
If we're scared to introduce new range of bugs, hard to understand stuff, maybe let's just drop it.

I think it would be nice to merge the PR, but it just doesn't make much sense to me that a default, always-defined value like `max_length=20` would overwrite something that's passed via the `logits_processor`. So instead of dropping the PR we can just ensure that passed `logits_processor` and `stopping_criteria` that are passed have priority which is intuitive and sensible to me. 

So, you think, we should

```python
if logits_processor is None:
   logist_processort = self._get_logits_process(...)
```
instead ?

Make sense.


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/master/CONTRIBUTING.md) are likely to be ignored.

Leaving it as closed for now - reopening in case the community expresses interest in this PR again...

Thanks a lot for taking this over @lvwerra ! Let me know if you need any help with the remaining tests

Superseeded by https://github.com/huggingface/transformers/pull/14779#issuecomment-997914237