[ModelOpt] Support Qwen3.5/3.6 VLM quantized prefix mapping

[meenchen]

Purpose

Add Qwen3.5/3.6 VLM checkpoint prefix mapping for ModelOpt mixed-precision checkpoints.

Qwen VLM checkpoints can export language model tensors and quantization config entries with wrapper prefixes such
as model.language_model.* or language_model.model.*, while vLLM constructs the text model modules under
normalized internal prefixes such as model.* / lm_head.*.

This PR adds the required mapping so:

• Qwen3.5/3.6 VLM checkpoint weights load into the expected vLLM module names.
• ModelOpt quantized_layers entries resolve correctly when wrapper prefix ordering differs.
• Nested VLM lm_head prefixes resolve to top-level lm_head quantization metadata when lm_head quantization support is present. (related PR: Add LM head quantization support for ModelOpt #42124)

Test Plan

• Run smoke generation with a Qwen3.6 VLM ModelOpt mixed-precision checkpoint.
• Verify ModelOpt quantized_layers resolves the expected quantized modules when checkpoint prefixes use
  model.language_model.* / language_model.model.*.
• Verify text-only loading works with Qwen VLM architecture override and disabled multimodal inputs.

Test Result

Tested locally on the cluster as part of the Qwen3.6 ModelOpt checkpoint smoke flow. The checkpoint loads with the
VLM prefix mapping, resolves quantized layer metadata, and generates text successfully.

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[gemini-code-assist]

Code Review

This pull request enables quantization for the language model head in Nemotron-H and Qwen 3.5 models. Key changes include updating the ModelOpt quantization framework to support ParallelLMHead, implementing reciprocal round-trip logic for NVFP4 W4A16 to ensure bit-identical results with other backends, and introducing a prefix mapping utility to resolve quantization configurations across different model naming conventions. The PR also adds unit tests for these features and fixes a weight loading issue for scalar scales. Review feedback identifies missing prefix mappings required for Qwen VLM checkpoints to correctly resolve quantization methods.

[claude]

Claude Code Review

This pull request is from a fork — automated review is disabled. A repository maintainer can comment @claude review to run a one-time review.

[mergify]

Hi @meenchen, the pre-commit checks have failed. Please run:

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[mergify]

Hi @meenchen, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

Tip

Is mypy failing?

mypy is run differently in CI. If the failure is related to this check, please use the following command to run it locally:

# For mypy (substitute "3.10" with the failing version if needed)
pre-commit run --hook-stage manual mypy-3.10

[pavanimajety]

Thanks for the PR!