[Model] Nemotron-H 3.5: quantized LM head and MTP compressed-tensors fix

[askliar]

Purpose

Two model-side fixes needed to run Nemotron-H 3.5 on the released NVFP4 checkpoints.

Part 2/3 of splitting #43333 (the combined B12x + Nemotron-3.5 + Qwen3.5 PR). Logically depends on the LM-head quant routing in part 1/3 (#43341) — once that routing exists, quant_config=self.quant_config on ParallelLMHead actually does something useful.

Changes

• NemotronHForCausalLM, NemotronHMTP: pass quant_config to ParallelLMHead so quantized LM heads load correctly.
• NemotronHMTP: extend compressed_tensors_config.ignore with the per-expert MTP linears (gate_proj / up_proj / down_proj) before constructing self.layers. Those linears are BF16 in the released nvidia/Nemotron-H-3.5-* compressed-tensors checkpoints; without this, the FusedMoE quant-method dispatch fails with KeyError during weight load (same failure mode as [Bugfix] Extend compressed-tensors ignore for Qwen3.5 MTP experts #41994 for Qwen3.5 MTP, different model).

Relation to existing open PRs (overlap disclosure)

• Add LM head quantization support for ModelOpt #42124 (Add LM head quantization support for ModelOpt) already includes the one-line quant_config=self.quant_config for NemotronHForCausalLM.lm_head. My PR includes the same one-liner plus the matching one for NemotronHMTP.lm_head. If Add LM head quantization support for ModelOpt #42124 lands first, drop the NemotronHForCausalLM hunk on rebase.

Unique to this PR:

• NemotronHMTP quant_config plumbing for lm_head (Add LM head quantization support for ModelOpt #42124 does not touch nemotron_h_mtp.py).
• NemotronHMTP compressed-tensors ignore extension for per-expert MTP linears — analogous to [Bugfix] Extend compressed-tensors ignore for Qwen3.5 MTP experts #41994 but for Nemotron-H. No open PR I found covers this for Nemotron.

Test Plan

• End-to-end serve Nemotron-H 3.5 (modelopt NVFP4 W4A4) — exercises quantized LM head + MTP.
• End-to-end serve Nemotron-H 3.5 (compressed-tensors nvfp4-pack-quantized W4A16) — exercises MTP ignore-list extension.

Fill in actual test results before review per AGENTS.md.

AI assistance

This change was developed with AI assistance (Claude). Every changed line was reviewed locally by the submitter.

🤖 Generated with Claude Code

[gemini-code-assist]

Code Review

This pull request updates the NemotronH and NemotronH-MTP models to support quantization configurations by passing the quantization config to the language model head. Additionally, it implements logic in the NemotronH-MTP model to automatically exclude specific per-expert linear layers from compressed-tensors quantization when they are expected to remain in BF16 format. I have no feedback to provide.