[New Model] support new model ovis2.6

[myselvess]

Purpose

1. support new model ovis2.6 (https://huggingface.co/AIDC-AI/Ovis2.6-30B-A3B).
2. fixed a bug in Ovis2_5MultiModalProcessor._get_prompt_updates to support input prompt_token_ids directly.
3. fixed a bug where post_layernorm was not needed for the last_hidden_state in Siglip2VisionTransformer.

ps: Ovis2.6-2B is coming soon

Test Plan

Test Result

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Essential Elements of an Effective PR Description Checklist

• The purpose of the PR, such as "Fix some issue (link existing issues this PR will resolve)".
• The test plan, such as providing test command.
• The test results, such as pasting the results comparison before and after, or e2e results
• (Optional) The necessary documentation update, such as updating supported_models.md and examples for a new model.
• (Optional) Release notes update. If your change is user facing, please update the release notes draft in the Google Doc.

[mergify]

Documentation preview: https://vllm--34426.org.readthedocs.build/en/34426/

[gemini-code-assist]

Code Review

This pull request introduces support for the new ovis2.6 model by adapting and reusing the existing ovis2.5 implementation. It also includes two bug fixes: one in Ovis2_5MultiModalProcessor to correctly handle prompt_token_ids, and another in Siglip2VisionTransformer where an unnecessary post_layernorm is removed. The changes are well-structured. My review includes one suggestion to replace a magic number with a constant-derived expression for better code clarity and maintainability.

[Isotr0py]

Hmmm, I'm a bit worried that these changes to support Ovis2.6 will also break existing Ovis2.5 support...

[Isotr0py]

Can you also update tests/models/registry.py? If Ovis-2.6-2B is unavailable yet, we can set is_available_online=False for it.

[Isotr0py]

Will Ovis2.5's tokenizer also be updated? I'm a bit worried that this change will break Ovis2.5 support. Especially Ovis2.5 has various backbones with different tokenizer.