chore: Improve checkpoint loading error messages with common issue and a fix

[ahmadki]

What does this PR do ?

Improves checkpoint loading error messages

Summary by CodeRabbit

• Bug Fixes
  • Improved error messaging for Megatron policy checkpoint loading failures. When issues occur, users now receive detailed guidance on common causes and troubleshooting steps, including checkpoint location information.

_{✏️ Tip: You can customize this high-level summary in your review settings.}

[coderabbitai]

📝 Walkthrough

Walkthrough

Enhanced error handling for YAML config loading in the Megatron policy worker. When loading a pretrained checkpoint fails, the exception is caught and augmented with detailed diagnostic information, including common causes like checkpoint version mismatches, guidance for resolution, and the checkpoint file location before re-raising.

Changes

Cohort / File(s)	Summary
Error handling enhancement nemo_rl/models/policy/megatron_policy_worker.py	Wrapped YAML config loading in try/except to catch and augment exceptions with diagnostic notes about potential HF→mcore checkpoint version mismatches, remediation steps, and checkpoint location.

Estimated code review effort

🎯 2 (Simple) | ⏱️ ~8 minutes

• Single file, localized change with straightforward error handling logic
• Exception augmentation is linear and deterministic
• No complex branching or multi-file dependencies to verify

Suggested labels

CI:L1

Suggested reviewers

• yaoyu-33
• terrykong

Pre-merge checks and finishing touches

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	You can run @coderabbitai generate docstrings to improve docstring coverage.

✅ 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 clearly describes the main change: improving checkpoint loading error messages with details about common issues and fixes, which aligns with the changeset's focus on wrapping YAML config loading in try/except and augmenting exceptions with helpful context.
Test Results For Major Changes	✅ Passed	Changes are minor error handling improvements with no impact on features, APIs, or logic; test results documentation is only required for major changes.

✨ Finishing touches

• 📝 Generate docstrings

🧪 Generate unit tests (beta)

• Create PR with unit tests
• Post copyable unit tests in a comment
• Commit unit tests in branch ahmadki/mbridge_checkpoint_error

---

📜 Recent review details

Configuration used: Path: .coderabbit.yaml

Review profile: CHILL

Plan: Pro

📥 Commits

Reviewing files that changed from the base of the PR and between 1c371a9 and 7307f7c.

📒 Files selected for processing (1)

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

⏰ Context from checks skipped due to timeout of 90000ms. You can increase the timeout in your CodeRabbit configuration to a maximum of 15 minutes (900000ms). (3)

• GitHub Check: Lint check
• GitHub Check: Post automodel integration comment / Comment on PR
• GitHub Check: Post submodule check comment / Comment on PR

🔇 Additional comments (1)

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

536-552: The code is compatible with the project's Python version requirements. No changes needed.

The project specifies requires-python = ">=3.12" in pyproject.toml, and since the add_note() method was introduced in Python 3.11, it is fully supported. The exception handling at lines 536-552 is correct as-is, and the suggested alternative approach is unnecessary.

Likely an incorrect or invalid review comment.

Tip

📝 Customizable high-level summaries are now available in beta!

You can now customize how CodeRabbit generates the high-level summary in your pull requests — including its content, structure, tone, and formatting.

• Provide your own instructions using the high_level_summary_instructions setting.
• Format the summary however you like (bullet lists, tables, multi-section layouts, contributor stats, etc.).
• Use high_level_summary_in_walkthrough to move the summary from the description to the walkthrough section.

Example instruction:

"Divide the high-level summary into five sections:

1. 📝 Description — Summarize the main change in 50–60 words, explaining what was done.
2. 📓 References — List relevant issues, discussions, documentation, or related PRs.
3. 📦 Dependencies & Requirements — Mention any new/updated dependencies, environment variable changes, or configuration updates.
4. 📊 Contributor Summary — Include a Markdown table showing contributions:
   | Contributor | Lines Added | Lines Removed | Files Changed |
5. ✔️ Additional Notes — Add any extra reviewer context.
   Keep each section concise (under 200 words) and use bullet or numbered lists for clarity."

Note: This feature is currently in beta for Pro-tier users, and pricing will be announced later.

---

Thanks for using CodeRabbit! It's free for OSS, and your support helps us grow. If you like it, consider giving us a shout-out.

❤️ Share

• X
• Mastodon
• Reddit
• LinkedIn

_{Comment @coderabbitai help to get the list of available commands and usage tips.}