[Model] Gemma4: robust quantized MoE expert weight loading

[yueshen2016]

Summary

• Add ignore_suffixes to gracefully skip unrecognized quantization scale parameters (e.g. weight_scale, input_scale) instead of raising KeyError in the fallthrough path
• Add return_success=True to weight_loader calls so FusedMoE can signal rejection and the loader can try the next expert mapping or skip gracefully
• Track is_expert_weight to skip expert keys not mapped to the current rank (distributed/EP scenarios)
• Guard against missing params in the fallthrough else block with name not in params_dict check

These changes match the robust loading pattern used by Qwen3 MoE (qwen3_moe.py lines 607-668) and ensure reliable loading of quantized (ModelOpt NVFP4/FP8, GPTQ) Gemma4 MoE checkpoints where per-expert scale tensors may not map to local parameters.

Context: When quantizing Gemma4 26B-A4B with ModelOpt, the exported checkpoint contains per-expert scale keys like experts.0.gate_proj.input_scale. The current _weight_iterator and expert_params_mapping (trailing dot/underscore pattern + re.sub remap) correctly handle these, but the loading loop lacks robustness: if a mapped scale name doesn't exist in params_dict, the code falls through to direct load and crashes with KeyError.

Test plan

• Quantize google/gemma-4-26B-A4B-it with ModelOpt NVFP4 (hf_ptq.py --qformat nvfp4_mlp_only)
• Serve with vllm serve <path> --quantization modelopt — should load without errors
• Verify text generation produces coherent output
• Verify unquantized Gemma4 checkpoint loading still works (regression test)

🤖 Generated with Claude Code

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[gemini-code-assist]

Code Review

This pull request refactors the load_weights method in gemma4.py to enhance the handling of various weight types, particularly for expert weights and quantization parameters. It introduces an ignore_suffixes tuple to manage skipping specific parameter suffixes during weight loading. The review feedback points out a redundancy where an explicit check for .bias can be removed, as it is already covered by the ignore_suffixes mechanism.

[gemini-code-assist]

This check for .bias is redundant. The ignore_suffixes tuple, defined on line 1286, already includes ".bias". Therefore, the check on lines 1389-1390 fully covers this case. You can remove these lines to avoid code duplication.