Generate: documented function to compute the transition scores
# What does this PR do?

Fixes #18616; Addresses comments in #5164, #20008 (and in a few other issues that I've lost track of).

## The issue

A few users would like to have a simple function to obtain the transition scores (i.e. the logits for each selected token at generate time). This is very useful for exploring the generated contents and simplifies the construction of powerful color-coded interfaces (e.g. [this one](https://joel.tools/codegen/)). It is also commonly requested to compare our models against OpenAI's.

We had a function for that in PT, `compute_beam_transition_scores`, but it was unknown to most users. This is because it was limited to Beam-based approaches, was not in our documents, and had no examples.

## The solution

This PR upgrades the function above to a first-class citizen 🥇 :
1. Makes it compatible with all generation strategies (e.g. Sample)
2. Adds a flag to renormalize the logits before fetching the right ones, which is a frequent downstream use
3. Adds it to the documentation
4. Populates the docstring with examples for which I've got questions a few times (how to print the token probabilities and how to recompute the score of the sequences in beam search)

In the process, I've decided to update the name of the function (`compute_beam_transition_scores` -> `compute_transition_scores`), to match better what it does. Although this technically breaks the API, the function was not part of our documented functions and, given the number of related issues, I'd say it was mostly unknown.
_The documentation is not available anymore as the PR was closed or merged._<|||||>@sgugger merging as the failing test is a known flaky test (`tests/models/auto/test_modeling_auto.py::AutoModelTest::test_from_pretrained_dynamic_model_distant`)<|||||>Thanks! This function is super helpful for my use case.