fix idx issue

[saurabh111233212]

If you don't have this, you'll get a idx error during this line (2006 of grpo_fast.py):
collated_vllm_logprobs.append( collate_fn([per_device_packed_vllm_logprobs[idx] for idx in micro_range], 0) )

this only can occur when you set allow_world_padding True

example job of the error here
after the fix: job is here

[gemini-code-assist]

Summary of Changes

Hello @saurabh111233212, I'm Gemini Code Assist¹! I'm currently reviewing this pull request and will post my feedback shortly. In the meantime, here's a summary to help you and other reviewers quickly get up to speed!

This pull request resolves a critical 'idx' error that manifested when 'allow_world_padding' was set to 'True' in 'grpo_fast.py'. The issue stemmed from an inconsistency in data padding, where 'vllm_logprobs' were not being padded alongside other sequence components. The fix introduces dummy 'vllm_logprob' entries, filled with NaN values, to ensure all data structures maintain consistent lengths during the padding process, thereby preventing index out-of-bounds exceptions during data collation.

Highlights

• Error Fix: Addresses an 'idx' out-of-bounds error occurring in 'grpo_fast.py' when 'allow_world_padding' is enabled.
• Padding Consistency: Ensures that 'vllm_logprobs' are consistently padded with dummy entries (NaNs) when 'allow_world_padding' is active, matching the length of other padded sequences.
• Data Integrity: Prevents index out-of-bounds exceptions during data collation by maintaining consistent list lengths across all sequence components.

Using Gemini Code Assist

The full guide for Gemini Code Assist can be found on our documentation page, here are some quick tips.

Invoking Gemini

You can request assistance from Gemini at any point by creating a comment using either /gemini <command> or @gemini-code-assist <command>. Below is a summary of the supported commands on the current page.

Feature	Command	Description
Code Review	/gemini review	Performs a code review for the current pull request in its current state.
Pull Request Summary	/gemini summary	Provides a summary of the current pull request in its current state.
Comment	@gemini-code-assist	Responds in comments when explicitly tagged, both in pull request comments and review comments.
Help	/gemini help	Displays a list of available commands.

Customization

To customize Gemini Code Assist for GitHub experience, repository maintainers can create a configuration file and/or provide a custom code review style guide (such as PEP-8 for Python) by creating and adding files to a .gemini/ folder in the base of the repository. Detailed instructions can be found here.

Limitations & Feedback

Gemini Code Assist may make mistakes. Please leave feedback on any instances where its feedback is incorrect or counter productive. You can react with 👍 and 👎 on @gemini-code-assist comments. If you're interested in giving your feedback about your experience with Gemini Code Assist for Github and other Google products, sign up here.

You can also get AI-powered code generation, chat, as well as code reviews directly in the IDE at no cost with the Gemini Code Assist IDE Extension.

Footnotes

1. Review the Privacy Notices, Generative AI Prohibited Use Policy, Terms of Service, and learn how to configure Gemini Code Assist in GitHub here. Gemini can make mistakes, so double check it and use code with caution. ↩

[gemini-code-assist]

Code Review

This pull request correctly fixes an IndexError that occurs when allow_world_padding is enabled. The issue was caused by missing padding for vllm_logprobs when creating dummy sequences. The fix adds the necessary padding for packed_sequences.vllm_logprobs, ensuring its length is consistent with other padded sequence data. The change is minimal, targeted, and effectively resolves the bug. The code looks good.

[finbarrtimbers]

@gemini-code-assist is there a way we can make this code unit testable that would let us reproduce the bug in a unit test?

[gemini-code-assist]

That's an excellent question! Reproducing bugs with unit tests is a best practice for ensuring code stability and preventing regressions. For this specific idx error related to allow_world_padding and vllm_logprobs, it should indeed be possible to create a unit test.

Here's a general approach you could take:

1. Isolate the Data Preparation Logic: The core of the issue lies in the data_preparation_thread function, specifically where vllm_logprobs were not being padded consistently with other sequence components when allow_world_padding was True.

2. Simulate Input Conditions: Create a minimal set of input data that would lead to a shortfall (i.e., needing padding) when allow_world_padding is enabled. You'd need to mock or create simplified versions of packed_sequences and other relevant data structures.

3. Trigger the Bug (Pre-Fix):

   • Scenario 1 (Pre-fix data_preparation_thread): Call the data_preparation_thread function with allow_world_padding=True and inputs that would result in padding being applied to other sequences but not to vllm_logprobs. Then, attempt to access vllm_logprobs in a way that would trigger the idx error (e.g., iterating over them with an index that would be out of bounds due to length mismatch). You would assert that an IndexError or similar exception is raised.

4. Verify the Fix (Post-Fix):

   • Scenario 2 (Post-fix data_preparation_thread): Call the data_preparation_thread function with the fixed logic (where dummy_vllm_logprob is appended). Ensure that vllm_logprobs now has the correct padded length. Then, pass this prepared data to the part of the train function that processes mb_vllm_logprobs (where DIFF 1 is located). Assert that no IndexError occurs and that the mean_diff and mean_reverse_kl calculations correctly handle the NaN values introduced by the dummy logprobs.

This approach would allow you to confirm that the padding inconsistency is resolved and that subsequent calculations gracefully handle the padded NaN values, effectively reproducing and verifying the fix for the bug in a controlled unit test environment.