[Bugfix] Make Gemma 4 suppress-token masking CUDA-graph safe

[dumko2001]

Purpose

Gemma 4 CUDA graph capture used a Python list of suppress-token IDs and
advanced-index assignment with a Python scalar. Both can trigger host-to-device
copies during capture. Store the IDs as a device buffer and use index_fill_
for graph-safe masking in the MTP and multimodal paths.

Fixes #48503

Related work: PR #48509 fixed only the list indexing in gemma4_mtp.py and
left the scalar-assignment copy in place; its author closed it in favour of
PR #48515. #48515 fixed both copy sources with register_buffer and
index_fill_, but created the tensor without a device=, so the buffer
starts on CPU and depends on a later module .to(device) that the MTP
drafter load path does not perform. It was closed unmerged with no maintainer
comment. This change creates the buffer directly on
vllm_config.device_config.device. The follow-up commit restores the same
device-buffer initialization in gemma4_unified.py; its __init__ is rebuilt
without calling super(), so it gets its own device-buffer initialization. AI
assistance was used; I reviewed every changed line and the GPU result.

Test Plan

uv tool run ruff check repro/gpu_validate_gemma4_suppress_tokens.py
uv run --no-project python -m py_compile \
  repro/gpu_validate_gemma4_suppress_tokens.py
python repro/gpu_validate_gemma4_suppress_tokens.py

The CUDA validation ran with the stock wheel and then with these files
overlaid:

vllm/model_executor/models/gemma4_mtp.py
vllm/model_executor/models/gemma4_mm.py

The script inspects the imported source before running synthetic CUDA graph
capture and replay.

Added CPU unit coverage for suppress-token masking:

PYTHONPATH=. .venv/bin/python -m pytest --noconftest \
  tests/model_executor/test_gemma4_suppress_tokens.py -q

Test Result

The patched module was confirmed by source inspection. The device-buffer
index_fill_ path captured and replayed successfully, and the masked columns
were -inf while the other logits remained unchanged. The original Python
list pattern failed during capture as expected.

GEMMA4_VARIANT: PATCHED
DEVICE_BUFFER_CAPTURE: PASS
PYTHON_LIST_CAPTURE: FAIL (expected)
PASS: imported Gemma4 helper uses device-buffer index_fill_

The full Gemma model startup and generation path were not run.

---

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