[5/N] Initialize MM components in context managers (Q-Z)

[DarkLight1337]

Purpose

Part of #32631

• Skipped Qwen-VL and Whisper as the MM and LM parts are in the same child module.
• Skipped Terratorch impl as there is only MM component but not LM component.
• Skipping Transformers impl for now; we may revisit this after adding video support ([Model] Add video input support for transformers modeling backend #30680) and getting a better idea of how to target the modules to skip.

Test Plan

Test Result

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[gemini-code-assist]

Code Review

This pull request refactors the initialization of multimodal components across several models (Qwen2-Audio, Qwen3-Omni-MoE-Thinker, Siglip, SkyworkR1V, Tarsier, Ultravox, Voxtral) by introducing _mark_tower_model and _mark_language_model context managers. This change enhances modularity and allows for more flexible management of model sub-components, particularly for scenarios like multimodal encoder-only modes where certain parts of the model might be skipped. The removal of explicit get_language_model methods and some assert statements indicates a shift towards a more streamlined and potentially more robust initialization pattern, where the presence of components is either guaranteed by design or handled gracefully within the new context-managed blocks. The additions of getattr checks for deepstack_input_embeds in qwen3_omni_moe_thinker.py are good for ensuring robustness when components might be conditionally initialized.