[khluu]

Purpose

Fix Gemma checkpoint loading after #51665 when a config declares tied word embeddings but the checkpoint contains a physical lm_head.weight.

Checkpoint inspection deliberately flips tie_word_embeddings to False so the physical head can be compared with the input embeddings after loading. GemmaForCausalLM did not construct an lm_head, so GPTQ Gemma checkpoints failed with There is no module or parameter named 'lm_head'. This change constructs the output head, ties it to the input embeddings for normally tied configs, and uses it for logits.

Duplicate-work check: searched open PRs and issues for Gemma lm_head weight, tie_word_embeddings Gemma, and the affected TechxGenus/gemma-1.1-2b-it-GPTQ model. No open change addresses this Gemma loader failure; #30412 is scoped to GGUF mapping.

AI assistance was used to investigate the regression, implement the change, and draft the test. The submitting human reviewed the changed lines and validation results.

Test Plan

• Focused CPU regression for checkpoint-driven Gemma untying.
• Pre-commit hooks on both changed files.
• Exact H200 Quantized Models Buildkite job with TechxGenus/gemma-1.1-2b-it-GPTQ at this PR head.

Test Result

• .venv/bin/python -m pytest tests/models/language/generation/test_gemma.py -q -k checkpoint_lm_head_can_override_tied_config: 1 passed, 3 deselected.
• uvx pre-commit run --files vllm/model_executor/models/gemma.py tests/models/language/generation/test_gemma.py: all applicable hooks passed, including ruff and mypy.
• Model evaluation: Buildkite #84894 passed at exact PR commit d3494fc2 on H200 (h200-ci-3). The complete Quantized Models suite finished 22 passed, 32 skipped, 0 failed in 33m16s of pytest time (34m27s job wall time). Both formerly failing TechxGenus/gemma-1.1-2b-it-GPTQ cases passed: test_models[5-32-half-model2] and test_models[5-32-bfloat16-model2]. Proposed fix is validated; the incident remains open until merge and a post-merge main exact-job pass.

[claude]

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[khluu]

/ci run

[github-actions]

✅ Triggered Buildkite CI #84897 for commit d3494fc2281b.