docs(ultra3): update quantization recipe and precision table

[yueshen2016]

Summary

Replace the generic NVFP4 description in docs/nemotron/ultra3/quantization.md with the actual hybrid recipe used for the released Nemotron 3 Ultra checkpoint, and align the precision settings table with super-nvfp4-max-calib.yaml.

Changes

• Hybrid checkpoint description: clarify that the released checkpoint is NVFP4 W4A4 for routed experts, FP8 per-tensor for shared experts and Mamba mixer.{in,out}_proj, FP8 KV cache, BF16 everywhere else (attention, latent MoE, MTP, lm_head, embeddings, Mamba conv1d/SSM).
• Precision table: updated to reflect the recipe layer-by-layer; old table marked attention GEMMs and Mamba GEMMs as NVFP4, which is not what the recipe actually applies.
• Commands section: replaces the Recipe Execution section. Updates the quantize command to pass the YAML recipe path via --export-quant-cfg instead of the built-in mamba_moe_nvfp4_conservative shorthand (the released recipe is a YAML file, not a built-in name).
• Memory-reduction figure: tightened from ~4× to the measured ~3.3× (1.1 TB BF16 → 331 GB hybrid).
• Parameter count: tightened to ≈560B (inferred from the BF16 checkpoint size).
• Removed the "Quantization Configurations" table listing four built-in shorthand names — the released recipe is a YAML file and the table is no longer the authoritative list.
• Removed the "Parallelism Configuration" subsection inside Infrastructure — redundant with the --pp/--tp/--ep flags shown in each command.
• Kept Infrastructure components table and Reference section unchanged.

Test plan

• Verified the new precision table matches the recipe YAML's enable list (*mlp.experts* → NVFP4, *mlp.shared_experts* and *mixer.{in,out}_proj* → FP8, everything else disabled).
• Verified the commands match examples/quantization/{quantize,ptq_generate,export}.py in Megatron-Bridge.
• Verified the super-nvfp4-max-calib.yaml recipe path and that PTQ + export + sanity-prompt round-trip works end-to-end on this checkpoint.

🤖 Generated with Claude Code