feat(distill): support padded vocabulary tails

[efegokdemir]

What does this PR do?

Type of change: new feature and regression tests

Adds an explicit vocabulary alignment option to LogitsDistillationLoss, addressing #2489. Distillation between models with the same token IDs but padded vocabulary tails can now restrict both logits tensors to a shared prefix before computing KL divergence.

Usage

criterion = mtd.LogitsDistillationLoss(vocab_size=151936)

Use vocab_size only when the retained token IDs are aligned. It does not remap token IDs or account for tokens outside the selected range.

Testing

• python3 -m compileall -q modelopt/torch/distill/losses.py tests/unit/torch/distill/test_distill.py: passed
• git diff --check: passed
• pytest -q tests/unit/torch/distill/test_distill.py -k logits_distillation: not run because pytest and PyTorch are not installed locally; installing the full ML test stack was avoided because of workspace disk constraints.
• No duplicate open PR was present when checked with gh pr list -R NVIDIA/Model-Optimizer --state open --search 2489 in:body.

Before your PR is Ready for review

• Backward compatible: yes
• Copied code or added a dependency: N/A
• Necessary tests added: yes
• Changelog updated: N/A
• Claude approval: N/A

Additional Information

Closes #2489.

AI assistance was used for investigation, implementation, testing setup, and this PR description. I reviewed the complete diff and accept responsibility for its correctness, security, maintainability, and license compliance. The commit is DCO-signed and SSH-signed.

Summary by CodeRabbit

• New Features

  • Added support for distilling models with aligned token IDs and differently sized vocabularies.
  • Users can specify a positive vocabulary size to restrict distillation to the shared vocabulary range.
  • Added guidance for configuring vocabulary sizes during model distillation.

• Bug Fixes

  • Added validation for invalid vocabulary sizes and insufficient logits dimensions.

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📝 Walkthrough

Walkthrough

LogitsDistillationLoss now supports truncating aligned teacher and student vocabularies to a configured vocab_size. It validates the value and logits dimensions. Documentation and tests cover the new behavior.

Changes

Vocabulary truncation

Layer / File(s)	Summary
Loss configuration and validation modelopt/torch/distill/losses.py, tests/unit/torch/distill/test_distill.py	LogitsDistillationLoss accepts an optional positive vocab_size and rejects invalid values, including zero, negative, boolean, and non-integer values.
Logit truncation and coverage modelopt/torch/distill/losses.py, tests/unit/torch/distill/test_distill.py, docs/source/guides/4_distillation.rst	When configured, the loss validates both logits dimensions and truncates both tensors to the first vocab_size entries. Tests and documentation cover padded teacher vocabularies and oversized configurations.

Priority: ➖ Normal

Estimated code review effort: 2 (Simple) | ~10 minutes

Change: Feature · Severity of issue fixed: Medium

Merge Risk: ⚪ Minimal · up to b2911

The loss now supports aligned vocabularies with padded tails while rejecting invalid configurations; the documented behavior and focused tests indicate the change is ready to merge.

🚥 Pre-merge checks | ✅ 5 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 71.43% which is insufficient. The required threshold is 80.00%. Docstring coverage is scoped to functions touched by this diff. Analyzed 7 functions across 2 files.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (5 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title clearly and concisely describes the main change: adding support for padded vocabulary tails during distillation.
Linked Issues check	✅ Passed	The PR satisfies the coding requirements in issue #2489. LogitsDistillationLoss accepts vocab_size, validates positive non-boolean integers, checks both logits dimensions, and truncates both tenso…
Out of Scope Changes check	✅ Passed	The changes stay within issue #2489. The source change implements vocabulary alignment, the tests verify that behavior and its validation, and the documentation explains the supported usage and limits…
Security Anti-Patterns	✅ Passed	PASS. The pull request changes only modelopt/torch/distill/losses.py, tests, and documentation. Added-line scanning found no torch.load(..., weights_only=False), allow_pickle=True, hardcoded `tr…

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Inline comments:
In `@modelopt/torch/distill/losses.py`:
- Around line 51-53: Update the vocab_size validation before assigning
self._vocab_size to reject bool and any non-int value, while continuing to allow
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