fix: Fix DTensor slice crash after PyTorch 2.9 bump

[zpqiu]

What does this PR do ?

With DTensor on PyTorch 2.9, __getitem__ slice indexing can hit aten.alias.default (no sharding strategy registered) and crash.
Use narrow to avoid the alias/view path while keeping the same semantics.

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Summary by CodeRabbit

• Refactor
  • Improved internal implementation of sequence packing logic for more explicit bounds calculation and enhanced reliability.

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📝 Walkthrough

Walkthrough

Refactors logit slice derivation in the sequence packing loop by replacing a Python slice object with explicit start/end indices and tensor.narrow, maintaining equivalent functionality with more explicit bounds calculation.

Changes

Cohort / File(s)	Summary
Logit slicing refactoring nemo_rl/algorithms/loss_functions.py	Replaced logit_slice_idxs slice object with explicit logit_start, logit_end, and logit_length indices; uses tensor.narrow for next-token logits extraction within sequence packing loop

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

Suggested labels

CI:L1, r0.5.0

Suggested reviewers

• terrykong

Pre-merge checks and finishing touches

✅ Passed checks (4 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.
Test Results For Major Changes	✅ Passed	PR contains a minor, localized bug fix replacing slice indexing with tensor.narrow() to address PyTorch 2.9 DTensor compatibility.
Title check	✅ Passed	The title accurately describes the main change: fixing a DTensor slice crash caused by PyTorch 2.9, which matches the PR's primary objective of replacing slice indexing with tensor.narrow to avoid the alias/view path.

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Actionable comments posted: 0

🧹 Nitpick comments (1)

nemo_rl/algorithms/loss_functions.py (1)

925-930: Fix correctly addresses the DTensor crash.

The change from slice indexing to tensor.narrow is correct and preserves the original semantics while avoiding the aten.alias.default issue with DTensor on PyTorch 2.9.

The explicit calculation of logit_start, logit_end, and logit_length improves code clarity.

💡 Optional: Add explanatory comment

Consider adding a brief comment explaining why narrow is used instead of slice indexing to prevent future refactoring back to the problematic pattern:

 logit_start = seq_start // cp_size
 logit_end = (seq_start + padded_seq_lengths[seq_idx]) // cp_size
 logit_length = logit_end - logit_start
+# Use narrow instead of slicing to avoid DTensor crash with aten.alias.default on PyTorch 2.9+
 next_token_logits_slice = next_token_logits.narrow(
     1, logit_start, logit_length
 )

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• nemo_rl/algorithms/loss_functions.py

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