[Quant][MTP] Reuse packed FP8 logits inputs

[WoosukKwon]

Purpose

Fuse the sparse DeepSeek V3.2 / GLM-5.2 MTP layer's final all-reduce and RMSNorm
with packed FP8 quantization for its shared LM head.

The draft layer returns two deliberately different values:

• a packed QuantizedActivation for logits projection; and
• the post-final-norm BF16 hidden state recycled into the next MTP position.

The logits processor and both autoregressive speculators preserve and index this
contract without unpacking and requantizing it.

Dependencies

• [Model] Register sparse DeepSeek V3.2 MTP #51938: select/register the sparse MTP architecture
• [Quant] Let DeepGEMM consume prequantized activations #51939: DeepGEMM block-linear prequantized-input contract
• [Quant] Fuse all-reduce RMSNorm with packed FP8 #51942: all-reduce/RMSNorm/packed-FP8 producer

The branch contains only the seven-file MTP integration diff against main.
Until #51942 lands, isolated mypy reports the expected missing collective helper.

Duplicate-work check

No issue number was provided. I searched open PRs for DeepSeek V3.2 fused norm quant MTP. The only exact prior implementation was the superseded draft
#51936.

Tests

The portions independent of the missing prerequisite symbol pass directly:

.venv/bin/python -m pytest \
  tests/fusion/test_quant_activation_contract.py \
  tests/v1/worker/test_gpu_autoregressive_speculator.py -q
# 23 passed

On the integrated branch:

.venv/bin/python -m pytest tests/models/deepseek_v32/test_sequence_parallel.py -q
# 5 passed

All hooks other than the expected prerequisite-related mypy error pass.

Model evaluation

Two-node GB200, TP=8, C1, random 8K input / 1K output, MTP=3, ten measured
prompts, temperature 0:

Model	Overall	Position 0	Position 1	Position 2
zai-org/GLM-5.2-FP8	83.47%	90.83%	83.00%	76.57%
nvidia/GLM-5.2-NVFP4	89.82%	93.79%	90.15%	85.53%

Both runs completed all ten prompts without failures.

AI assistance disclosure

This change was developed with OpenAI Codex assistance. This is a blocked draft
PR; the human submitter must review every changed line and confirm they
understand and can defend the change before marking it ready.

[mergify]

This pull request has merge conflicts that must be resolved before it can be
merged. Please rebase the PR, @WoosukKwon.

https://docs.github.com/en/pull-requests/collaborating-with-pull-requests/working-with-forks/syncing-a-fork