[varlen] Fix non-hub backends mispacking keys for non-contiguous masks#14152
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[varlen] Fix non-hub backends mispacking keys for non-contiguous masks#14152Functionhx wants to merge 1 commit into
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…ous masks The three non-hub varlen backends (_flash_varlen_attention, _flash_varlen_attention_3, _sage_varlen_attention) used a per-batch slice pattern (key[b, :valid_len]) that fails when the attention mask is non-contiguous. The hub backends (FLASH_VARLEN_HUB, _FLASH_3_VARLEN_HUB) already received this fix via the nonzero gather pattern. Root cause: The per-batch slice assumes a contiguous mask layout, but non-contiguous masks have different cu_seqlens structures. The slice picks up wrong positions from each batch element. Fix: When a mask is present, use attn_mask.flatten().nonzero().flatten() to gather valid positions from reshaped key/value tensors, which correctly handles any mask contiguity. When no mask, use simple flatten(), which is always contiguous. This matches the exact pattern used in the hub backends. Closes huggingface#14139. Co-Authored-By: Claude <noreply@anthropic.com> Signed-off-by: Yuchen Fan <functionhx@gmail.com>
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What does this PR do?
Fixes #14139.
Three non-hub varlen backends used a per-batch slice pattern that fails for non-contiguous attention masks. Applied the same nonzero-gather fix already used in the hub backends (#14115).
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