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[bot]]

Summary of Changes

Hello @saurabh111233212, I'm Gemini Code Assist[^1]! 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.

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[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[bot]]

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.