[trainer] fix: maybe_filter_out_long_prompts on image and video

[firefighter-eric]

What does this PR do?

Add concise overview of what this PR aims to achieve or accomplish. Reference related GitHub issues and PRs that help with the review.

The filter_out_long_prompts function incorrectly used the messages variable when it should have used doc. This led to prompts with images or videos not being filtered correctly based on length.

Checklist Before Starting

• Search for similar PRs. Paste at least one query link here: ...
• Format the PR title as [{modules}] {type}: {description} (This will be checked by the CI)
  • {modules} include fsdp, megatron, sglang, vllm, rollout, trainer, ci, training_utils, recipe, hardware, deployment, ray, worker, single_controller, misc, perf, model, algo, env, tool, ckpt, doc, data
  • If this PR involves multiple modules, separate them with , like [megatron, fsdp, doc]
  • {type} is in feat, fix, refactor, chore, test
  • If this PR breaks any API (CLI arguments, config, function signature, etc.), add [BREAKING] to the beginning of the title.
  • Example: [BREAKING][fsdp, megatron] feat: dynamic batching

Test

For changes that can not be tested by CI (e.g., algorithm implementation, new model support), validate by experiment(s) and show results like training curve plots, evaluation results, etc.

API and Usage Example

Demonstrate how the API changes if any, and provide usage example(s) if possible.

# Add code snippet or script demonstrating how to use this

Design & Code Changes

Demonstrate the high-level design if this PR is complex, and list the specific changes.

The variable messages is of type list[dict], for example:

[
  {"type": "text", "content": "xxx"},
  {"type": "image", "content": "xxx"}
]

The variable doc is a dict, for example:

{
  "data_source": xxx.
  "prompt": xxx,
  "images": xxx,
}

We need to retrieve the image or video column from the dataset, load the actual images or videos, and then pass them into the tokenizer to obtain the sequence length.

Using messages here is incorrect — both the type and semantics are inappropriate. We should be using doc instead.

Checklist Before Submitting

Important

Please check all the following items before requesting a review, otherwise the reviewer might deprioritize this PR for review.

• Read the Contribute Guide.
• Apply pre-commit checks: pre-commit install && pre-commit run --all-files --show-diff-on-failure --color=always
• Add / Update the documentation.
• Add unit or end-to-end test(s) to the CI workflow to cover all the code. If not feasible, explain why: ...
• Once your PR is ready for CI, send a message in the ci-request channel in the verl Slack workspace.

[gemini-code-assist]

Code Review

This pull request fixes a bug in maybe_filter_out_long_prompts where image and video data were being popped from the wrong object, causing a crash when filtering long prompts with multimodal content. My review focuses on improving the fix by removing side effects, which enhances code correctness and maintainability.