Generate: post-generate config doctest fix
# What does this PR do?

Fixes doctests that were broken as a result of the `generation_config` PR merge.

Note: the failing pipeline test was fixing by adding a missing field to `gpt2`'s `generate_config.json` (which was created before these recent `generation_config` changes). See [this hub commit](https://huggingface.co/gpt2/commit/e7da7f221d5bf496a48136c0cd264e630fe9fcc8).
Just a question, so if users do things in the old way like

```
model.config.pad_token_id = model.config.eos_token_id
```
they might get different results before/after the generation config PR ..?<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>> Just a question, so if users do things in the old way like
> 
> ```
> model.config.pad_token_id = model.config.eos_token_id
> ```
> 
> they might get different results before/after the generation config PR ..?

`generate()` supports control from ad hoc model config changes (it has an extra check and handles differences [here](https://github.com/huggingface/transformers/blob/26dd041c6e45379141302e2d293ab4cd9cf805d4/src/transformers/generation/utils.py#L1131)), for retrocompatibility and ease of use. It is deprecated and will be removed soon.

Individual generation methods do not have this check, so they are not supporting control from ad hoc model config changes. The doctests in `main` are failing for this reason. It means that fixing the doctests can be done in two ways:
1. Add the same check to all individual generation methods
2. [current implementation in the PR] Change the doctest itself so they don't rely on ad hoc model config changes

I decided to follow 2. since we are deprecating it soon anyways AND calling the methods directly is an advanced feature (users should not be relying on side effects from the model config to support advanced functionality, it's a recipe for disaster 👀 ). 

WDYT? (cc @sgugger)<|||||>Agreed with option 2!