Revert DSv4 eager workspace reuse

[WoosukKwon]

Summary

Revert #49236 and restore allocator-backed temporary buffers in the DeepSeek V4 attention input-preparation path.

This removes the model-wide DeepseekV4EagerScratchPool, its output-buffer plumbing, and the out-parameter fused op added by #49236. Later changes on main are preserved.

Why

The model-wide scratch pool can reuse storage across layers and CUDA streams without the caching allocator's stream/event lifetime tracking. That allows a producer to overwrite shared workspace while a previous consumer may still be using it. Reverting restores the pre-#49236 allocation lifetime behavior until safe cross-stream synchronization is implemented.

Impact

#51430 moved Q projection/KV insertion, indexer preparation, and compressor preparation out of the narrow eager break and into the captured region. The allocator calls restored by this revert are therefore captured on the Model Runner V2 path, so this revert is not expected to give up the 3.9% TTFT improvement originally reported by #49236 there.

#52401 subsequently restored the wide eager region for Model Runner V1 because the narrow region corrupted V1 output. The allocation overhead remains eager on that path, so its TTFT impact is runner-dependent and has not been measured on the current tree.

Duplicate-work check

This is not duplicating an existing revert. I checked issue #45861 and searched open PRs for references to #49236 and for DeepSeek V4 workspace/eager-scratch reuse. No open PR is dedicated to reverting #49236.

Validation

• git diff --cached --check — passed before commit
• git diff --cached --name-only --diff-filter=ACMR -z | xargs -0 .venv/bin/pre-commit run --files — passed
• .venv/bin/python -m pytest tests/kernels/test_compressor_kv_cache.py tests/kernels/test_fused_deepseek_v4_qnorm_rope_kv_insert.py tests/kernels/test_fused_indexer_q_rope_quant.py -v — 240 passed, 11 skipped

Model evaluation

Not run: the DeepSeek V4 checkpoint is not available in the local Hugging Face cache. This PR remains a draft; DeepSeek-V4-Flash TP4 serving and GSM8K evaluation should be completed before it is marked ready.

AI assistance

OpenAI Codex was used to prepare the revert, resolve conflicts with later main changes, run validation, and draft this description. The human submitter must review every changed line and understand and defend the change end-to-end before marking the PR ready.

[claude]

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[mergify]

Hi @WoosukKwon, the pre-commit checks have failed. Please run:

uv pip install pre-commit>=4.5.1
pre-commit install
pre-commit run --all-files

Then, commit the changes and push to your branch.

For future commits, pre-commit will run automatically on changed files before each commit.

[WoosukKwon]

/ci run

[github-actions]

✅ Triggered Buildkite CI #84486 for commit 2cbd58585729.