[Proposal] Add HunYuan dense adapter (HunYuanDenseV1ForCausalLM)

[jlarson4]

Proposal

Add a TransformerBridge adapter for HunYuanDenseV1ForCausalLM (Tencent HunYuan dense family).

Motivation

Tencent ships small dense HunYuan models at 0.5B, 1.8B, 4B, and 7B, all runnable on a single consumer GPU. They fit interpretability compute budgets well, which makes the family high-leverage despite modest downloads.

Gap scan (2026-06-18): 13 models, ~114K downloads.

Pitch

The architecture is close to a standard dense decoder with GQA. Check the normalization and the dual fast/slow ("thinking") mode against the HF config while implementing.

• Claude Code users can scaffold with /add-model-support tencent/Hunyuan-1.8B-Instruct.
• Register at the four sites listed in contributing.md.
• Verify smallest-first: tencent/Hunyuan-0.5B, then tencent/Hunyuan-1.8B-Instruct.

Additional context

• Collection: https://huggingface.co/collections/tencent/hunyuan-dense-model
• Checkpoint: https://huggingface.co/tencent/Hunyuan-1.8B-Instruct
• Found via the hf_scraper architecture-gaps pass (2026-06-18).

Checklist

• I have checked that there is no similar issue in the repo (required)

[TensorCruncher]

I'd like to work on this issue please

[jlarson4]

Assigned! This one is all yours