Pipeline: no side-effects on `model.config` and `model.generation_config` 🔫 
Author: gante
Pull request: yes

# What does this PR do?

Fixes #33398 with two changes (and corresponding regression-preventing tests):
- [major change] Pipelines now hold their own `generation_config`. Modifications to `model.config` or `model.generation_config` were redirected to the pipeline's `generation_config`, and thus avoiding side-effects on the model.

```py
from transformers import pipeline, T5ForConditionalGeneration, AutoTokenizer
import tempfile

model = T5ForConditionalGeneration.from_pretrained("t5-small")
pipe = pipeline(
    model=model,
    tokenizer=AutoTokenizer.from_pretrained("t5-small", model_max_length=100),
    task="translation_en_to_de"
)

with tempfile.TemporaryDirectory() as tmpdirname:
    model.save_pretrained(tmpdirname)
    model_2 = T5ForConditionalGeneration.from_pretrained(tmpdirname)

# Prior to this PR, `model_2` != `model` because the pipeline initialization changed `model.config`!
```

- [minor change] in #32659, we started moving custom `generate` parameterization in `model.config` to `model.generation_config` when we called `model.save_pretrained()`. When we moved a parameter, `model.config` set that parameter to `None`. This `None` was not being properly ignored due to the legacy code path (parameterization in `model.config` takes precedence when the user never explicitly defined a `GenerationConfig`). After this PR, `None` fields are ignored by the generation config.

Comments:
Thanks! The changes make sense to me; I'd like to have @Rocketknight1's opinion as well

> The only question, though, is whether it would be better to just get rid of those side effects in the pipelines, rather than doing this copy to patch over the problem. Do you know how many pipelines do this?

@Rocketknight1 agreed 🙌 

You raised a good question -- in a rush to get the problem solved, I forgot to see the forest from the trees. From a quick check, only pipelines that call `generate` actually have this pattern, where `config` is modified to control the `generate` call. Good thing we have created a tool precisely to avoid this pattern, the `generation_config`! :D

As such, instead of doing this copy to patch over the problem, I think we can instead execute the following plan:
1. Text generation pipelines hold their own generation config
2. We apply the task-specific parameters (sometimes present in `model.config`, e.g. [here](https://huggingface.co/facebook/bart-large-cnn/blob/main/config.json#L55)) exclusively on the pipeline's `generation_config`. In other words, we keep the model's `config` and `generation_config` untouched
3. store `prefix` (an item that might present in the task-specific parameters but is not present in `GenerationConfig`) separately as a pipeline attribute
4. redirect all references to generation parameters to the pipeline's `generation_config`
5. we pass the pipeline's `generation_config` to `generate` in all pipelines

WDYT?

Yes, that sounds good, and a lot cleaner!

The docs for this PR live [here](https://moon-ci-docs.huggingface.co/docs/transformers/pr_33480). All of your documentation changes will be reflected on that endpoint. The docs are available until 30 days after the last update.

@Rocketknight1 v2 is ready for a review 🤗