axolotl/examples/nemotron-h/README.md

Nemotron-H (nvidia/NVIDIA-Nemotron-3-*)

Hybrid Mamba2 / Attention / MoE architecture (model_type: nemotron_h).

Model	Total params	Active params	Layers
NVIDIA-Nemotron-3-Super-120B-A12B-BF16	120B	~12B	88
NVIDIA-Nemotron-3-Nano-30B-A3B-BF16	30B	~3B	—

Requirements

pip install mamba-ssm causal-conv1d   # fast Mamba2 CUDA kernels

Architecture notes

• Three block types per layer: Mamba2 (selective SSM), Attention (sparse), MoE (mixture-of-experts).
• Only ~12 out of 88 blocks are attention layers (120B variant).
• MLP activation is relu2 via mlp_hidden_act (not the usual hidden_act).

LoRA kernel patches

All three LoRA Triton kernel patches must be disabled:

lora_qkv_kernel: false   # attention lives in NemotronHBlock.mixer, not layer.self_attn
lora_o_kernel: false     # same reason
lora_mlp_kernel: false   # relu2 (mlp_hidden_act) is not supported by lora_mlp_kernel

MoE expert weights

NemotronH experts store up_proj and down_proj as 3D nn.Parameter tensors (shape [num_experts, out_dim, in_dim]), not nn.Linear modules — there is no gate_proj. To fine-tune them alongside attention, use lora_target_parameters instead of lora_target_modules:

lora_target_parameters:
  - up_proj
  - down_proj

Limitations

• MoE Triton kernels: lora_mlp_kernel is not supported for NemotronH's MoE expert layers. The expert weights are 3D nn.Parameter tensors (not nn.Linear), which the Triton kernel does not support. Keep lora_mlp_kernel: false.
• Gradient checkpointing: Only supported when sample_packing: true. Without sample packing the upstream model marks supports_gradient_checkpointing = False.