[None][fix] SpecDec: Sampling seed fix

[IzzyPutterman]

Summary by CodeRabbit

• Bug Fixes
  • Improved speculative inference initialization with enhanced device tensor handling and flashinfer compatibility.

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[IzzyPutterman]

For when this PR is merged: flashinfer-ai/flashinfer#2432 and integrated into TRTLLM

[coderabbitai]

📝 Walkthrough

Walkthrough

The change modifies SpecWorkerBase to store seed and offset as optional torch tensors instead of integers, adds lazy initialization logic to place them on the correct device when FlashInfer is active, and increments seed before sampling.

Changes

Cohort / File(s)	Summary
Speculative Worker Initialization tensorrt_llm/_torch/speculative/interface.py	Changed seed and offset attributes from integer defaults (0) to optional torch tensors (None). Added lazy initialization of these tensors on the appropriate device and seed increment before passing to sampling.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~10 minutes

🚥 Pre-merge checks | ✅ 1 | ❌ 2

❌ Failed checks (2 warnings)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 50.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.
Description check	⚠️ Warning	The PR description is missing critical information: no functional description explaining the issue and solution, no test coverage details, and all checklist items are unfilled.	Add a Description section explaining what the seed fix addresses and why the change to Optional tensors is necessary, list relevant tests, and complete the PR checklist items.

✅ Passed checks (1 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title is specific and directly related to the main change: fixing a sampling seed issue in SpecDec, which aligns with the code changes to seed/offset initialization.

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[coderabbitai]

Actionable comments posted: 0

Caution

Some comments are outside the diff and can’t be posted inline due to platform limitations.

⚠️ Outside diff range comments (1)

tensorrt_llm/_torch/speculative/interface.py (1)

629-643: Optimize in-place tensor increment; offset behavior appears correct.

1. In-place tensor operation: self.seed += 1 allocates a new tensor. Use self.seed.add_(1) for true in-place modification.

2. Offset initialization: The offset tensor is initialized to [0] and remains unchanged throughout execution. This is intentional—the asymmetry between seed (which increments) and offset (which doesn't) reflects their different roles in FlashInfer's RNG system. This pattern is consistent throughout the codebase.

♻️ Suggested improvement for in-place increment

             if self.seed is None:
                 self.seed = torch.tensor([0], dtype=torch.int64, device=logits.device)
                 self.offset = torch.tensor([0], dtype=torch.int64, device=logits.device)
-            self.seed += 1
+            self.seed.add_(1)

[IzzyPutterman]

/bot run

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PR_Github #36287 [ run ] triggered by Bot. Commit: a09978f Link to invocation

[IzzyPutterman]

/bot run

[tensorrt-cicd]

PR_Github #36288 [ run ] triggered by Bot. Commit: 1254cc6 Link to invocation

[tensorrt-cicd]

PR_Github #36288 [ run ] completed with state SUCCESS. Commit: 1254cc6
/LLM/main/L0_MergeRequest_PR pipeline #28062 completed with status: 'FAILURE'

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[IzzyPutterman]

/bot run --disable-fail-fast

[tensorrt-cicd]

PR_Github #36305 [ run ] triggered by Bot. Commit: 5a194ab Link to invocation

[tensorrt-cicd]

PR_Github #36305 [ run ] completed with state SUCCESS. Commit: 5a194ab
/LLM/main/L0_MergeRequest_PR pipeline #28078 completed with status: 'FAILURE'

⚠️ Action Required:

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[IzzyPutterman]

/bot run --disable-fail-fast

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PR_Github #36317 [ run ] triggered by Bot. Commit: 5a194ab Link to invocation

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PR_Github #36317 [ run ] completed with state SUCCESS. Commit: 5a194ab
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