[Bugfix] Fix mrope in Transformers Backend

[zucchini-nlp]

Fixes the recently skipped test by creating image_grid_thw as batched fields. Otherwise they get an extra dimension and fail when preparing mrope positions in vLLM's model runners

Also, adds/rewrites some comments in processor code to keep them up to date. I tried to follow the base class apply() method, but transformers unfortunately cannot split the logic for placeholder and the rest into two. It will force us to call transformer utilities twice which is not very fast. So I am keeping it as is and simply adding comments

cc @hmellor as we have been talking about it internally

[gemini-code-assist]

Code Review

This pull request fixes an mrope issue in the Transformers backend by correctly configuring image_grid_thw and video_grid_thw as batched fields. It also includes several refactorings and cleanups, such as using a public API for setting attention implementation and removing unused code. The changes are generally good, but I've identified a critical issue where unguarded dictionary access could lead to a KeyError when processing text-only inputs in a multimodal model. I've provided a suggestion to make the code more robust.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @zucchini-nlp.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

issue has been fixed

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @zucchini-nlp.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork

[hmellor]

LGTM! Let's see what CI thinks