fix: Fix Fp8 sequence padding for PP>1 case

[guyueh1]

What does this PR do ?

This is a follow-up after #1569 , to fix the sequence length for PP>1 case.

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Summary by CodeRabbit

• Bug Fixes
  • Improved sequence packing parameter validation and alignment during training with packed sequences to ensure consistent padding enforcement.
  • Fixed sequence dimension calculation to use explicitly padded sequence lengths, enhancing correctness in forward and backward passes.

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📝 Walkthrough

Walkthrough

The changes enforce stricter padding constraints for packed sequences in Megatron models by asserting divisibility requirements, normalizing pad_packed_seq_to to multiples of a specified value, and using the explicitly padded sequence length for forward/backward computations.

Changes

Cohort / File(s)	Summary
Megatron sequence packing enforcement nemo_rl/models/megatron/common.py	Enforces pad_packed_seq_to divisibility via assertion in _pack_sequences_for_megatron; adds post-computation rounding in _get_pack_sequence_parameters_for_megatron to normalize pad_packed_seq_to to multiples of pad_packed_seq_to_multiple_of.
Policy worker sequence length alignment nemo_rl/models/policy/megatron_policy_worker.py	In train method, overrides seq_dim_size with pad_full_seq_to when active, aligning sequence length used in forward/backward calls and related parameters.

Estimated code review effort

🎯 3 (Moderate) | ⏱️ ~20 minutes

• Attention areas:
  • Verify that assertion in _pack_sequences_for_megatron doesn't break backward compatibility or existing call sites
  • Confirm the rounding logic in _get_pack_sequence_parameters_for_megatron correctly handles edge cases and preserves intended padding semantics
  • Ensure seq_dim_size override in megatron_policy_worker.py doesn't inadvertently affect downstream computations or loss calculations

Possibly related PRs

• fix: Fix the sequence padding for FP8 case #1569: Modifies the same Megatron sequence-packing padding functions and parameters (pad_packed_seq_to, pad_packed_seq_to_multiple_of, _get_pack_sequence_parameters_for_megatron).

Suggested reviewers

• youngeunkwon0405
• zpqiu
• terrykong

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Test Results For Major Changes	⚠️ Warning	PR contains medium-level changes to sequence padding logic for FP8 sequences with Pipeline Parallelism > 1, but lacks test results, performance metrics, or regression validation evidence.	Add test results validating the PP>1 FP8 sequence padding fix, numeric validation demonstrating no convergence regressions, performance benchmarks (before/after), and details of executed unit/functional tests.

✅ Passed checks (3 passed)

Check name	Status	Explanation
Description Check	✅ Passed	Check skipped - CodeRabbit’s high-level summary is enabled.
Title check	✅ Passed	The title 'fix: Fix Fp8 sequence padding for PP>1 case' is partially related to the changeset. It mentions FP8 sequence padding and PP>1, but the actual changes involve sequence padding validation and normalization logic in common.py and policy worker updates. The title accurately describes the domain but lacks specificity about the actual implementation changes (asserting divisibility, rounding up padding parameters).
Docstring Coverage	✅ Passed	Docstring coverage is 100.00% which is sufficient. The required threshold is 80.00%.

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📒 Files selected for processing (2)

• nemo_rl/models/megatron/common.py (2 hunks)
• nemo_rl/models/policy/megatron_policy_worker.py (1 hunks)

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• nemo_rl/models/megatron/common.py

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🔇 Additional comments (3)

nemo_rl/models/megatron/common.py (2)

87-89: LGTM! Proper contract validation.

The assertion enforces that pad_packed_seq_to must be a multiple of pad_packed_seq_to_multiple_of, which is essential for FP8 correctness with pipeline parallelism. This validates the contract between _get_pack_sequence_parameters_for_megatron (which normalizes the value) and this function.

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278-282: LGTM! Core fix for FP8 padding with PP > 1.

This normalization step ensures that when pipeline parallelism is used (PP > 1), the packed sequence length is rounded up to satisfy FP8 alignment requirements (multiples of 128 for blockwise, 16 for others). This is essential because all pipeline stages must see identical sequence lengths.

nemo_rl/models/policy/megatron_policy_worker.py (1)

1048-1049: LGTM! Correct sequence length override for PP > 1.

When sequence packing is enabled and pad_full_seq_to is not None (which occurs when PP > 1), this correctly overrides seq_dim_size with the normalized padded sequence length. This ensures that forward_backward_func (lines 1062-1082) receives consistent sequence lengths across all pipeline stages, which is required for pipeline parallelism correctness with FP8.

The pattern is also consistently applied in get_logprobs (line 1235) and get_topk_logits (line 1517).

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