Add default target_modules for Nemotron-H

What this does

Registers default target_modules for the nemotron_h model type so that LoraConfig() (and other PEFT methods) can be used on NVIDIA's Nemotron-3 hybrid Mamba + MoE + Attention models without users having to specify target_modules manually.

Defaults target the attention projections only (q_proj, k_proj, v_proj, o_proj). The Mamba mixer's in_proj / out_proj / conv1d are intentionally excluded because they belong to the Mamba block and are forbidden by the Mamba-architecture compatibility check added in #2562. nemotron_h is also added to mamba_model_types so the compatibility check applies to it.

Why

Without these defaults, calling LoraConfig() on a Nemotron-H model raises Please specify target_modules. NVIDIA's own NeMo Automodel LoRA cookbook for Nemotron-3 works around this by passing exclude_modules=["*.out_proj"] and bypasses PEFT for the inner LoRA application — extending PEFT to handle nemotron_h natively fixes the friction upstream.

What changed

src/peft/utils/constants.py — adds "nemotron_h" to four mappings, following the precedent established in #3136 (gemma4):

• TRANSFORMERS_MODELS_TO_LORA_TARGET_MODULES_MAPPING
• TRANSFORMERS_MODELS_TO_ADALORA_TARGET_MODULES_MAPPING
• TRANSFORMERS_MODELS_TO_VBLORA_TARGET_MODULES_MAPPING
• TRANSFORMERS_MODELS_TO_WAVEFT_TARGET_MODULES_MAPPING

src/peft/tuners/tuners_utils.py — adds "nemotron_h" to mamba_model_types in _check_lora_target_modules_mamba so the Mamba forbidden-module check (out_proj, conv1d) applies. This protects users who might explicitly try to target those names without realizing they belong to the Mamba mixer.

tests/test_custom_models.py — adds two tests under TestDefaultTargetModules:

1. test_default_target_modules_nemotron_h: verifies the constant lookup for all four mappings and asserts the defaults do not include forbidden Mamba modules.
2. test_nemotron_h_blocks_mamba_modules: verifies the Mamba compatibility check raises when a user explicitly targets out_proj on a model whose model_type is "nemotron_h".

Verified

Local syntax check passes (ast.parse on all three files). Test suite run pending environment install.

Related

• Precedent: Enh add default target modules for gemma4 and tests #3136 (gemma4 defaults)
• Mamba block constraint: Prevent applying LoRA to disallowed modules in Mamba-based architectures #2562