FA num splits option

[wdykas]

Description

I want to be able to control num splits in FA3. This exposes this argument

Type of change

• Documentation change (change only to the documentation, either a fix or a new content)
• Bug fix (non-breaking change which fixes an issue)
• New feature (non-breaking change which adds functionality)
• Breaking change (fix or feature that would cause existing functionality to not work as expected)
• Infra/Build change
• Code refactoring

Changes

Please list the changes introduced in this PR:

• Change A
• Change B

Checklist:

• I have read and followed the contributing guidelines
• The functionality is complete
• I have commented my code, particularly in hard-to-understand areas
• I have made corresponding changes to the documentation
• My changes generate no new warnings
• I have added tests that prove my fix is effective or that my feature works
• New and existing unit tests pass locally with my changes

[greptile-apps]

Greptile Overview

Greptile Summary

This PR exposes the num_splits parameter for FlashAttention v2 and v3 backends, allowing users to control memory optimization during attention computation.

Key Changes:

• Added optional num_splits parameter to DotProductAttention.forward() method
• Passes num_splits to both FlashAttention v2 and v3 backend implementations when provided
• Parameter is conditionally added to kwargs only when not None

Areas for Improvement:

• Missing parameter documentation in the docstring
• No version compatibility check for flash-attn (unlike other optional parameters like window_size and deterministic)
• No tests demonstrating the new functionality

Confidence Score: 4/5

• This PR is safe to merge with minor documentation improvements recommended
• The implementation correctly follows the existing pattern for optional parameters in FlashAttention backends. The changes are minimal and well-scoped. However, the score is not 5 due to: (1) missing parameter documentation, (2) lack of version compatibility checks that other optional parameters have, and (3) no accompanying tests. These are quality-of-life improvements rather than critical issues.
• No files require special attention - the implementation is straightforward and follows existing patterns

Important Files Changed

File Analysis

Filename	Score	Overview
transformer_engine/pytorch/attention/dot_product_attention/backends.py	4/5	Added num_splits parameter to FlashAttention forward method and passes it to both FA v2 and FA v3 backends when provided
transformer_engine/pytorch/attention/dot_product_attention/dot_product_attention.py	4/5	Added num_splits parameter to DotProductAttention forward signature and forwards it to FlashAttention backend

Sequence Diagram

sequenceDiagram
    participant User
    participant DotProductAttention
    participant FlashAttention
    participant FlashAttn_v2
    participant FlashAttn_v3

    User->>DotProductAttention: forward(query, key, value, num_splits=...)
    Note over DotProductAttention: New optional parameter
    DotProductAttention->>FlashAttention: forward(..., num_splits=num_splits)
    Note over FlashAttention: Check backend version
    alt Flash Attention v2
        FlashAttention->>FlashAttention: if num_splits is not None
        FlashAttention->>FlashAttention: fa_optional_forward_kwargs["num_splits"] = num_splits
        FlashAttention->>FlashAttn_v2: flash_attn_func(..., **fa_optional_forward_kwargs)
        FlashAttn_v2-->>FlashAttention: output
    else Flash Attention v3
        FlashAttention->>FlashAttention: if num_splits is not None
        FlashAttention->>FlashAttention: fa_3_optional_forward_kwargs["num_splits"] = num_splits
        FlashAttention->>FlashAttn_v3: flash_attn_func(..., **fa_3_optional_forward_kwargs)
        FlashAttn_v3-->>FlashAttention: output
    end
    FlashAttention-->>DotProductAttention: output
    DotProductAttention-->>User: output

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[greptile-apps]

style: Missing documentation for the num_splits parameter in the docstring. Add a parameter description explaining what this controls and when it should be used (e.g., "Controls the number of splits for FlashAttention computation. Used for memory optimization.")

[greptile-apps]

style: Verify that the flash-attn version supports num_splits parameter. Unlike other optional parameters (e.g., window_size has fa_utils.v2_3_plus check, deterministic has fa_utils.v2_4_1_plus check), this parameter is added without a version guard.

[greptile-apps]

style: Same as FA v2: verify flash-attn v3 supports num_splits to avoid potential TypeError