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ModernBERT FlexAttention #35423

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ModernBERT FlexAttention #35423

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What does this PR do?

This PR adds FlexAttention support for ModernBERT:

  • Combines sliding window and document masking to implement the alternating local/global attention pattern in ModernBERT
  • Mask creation is expensive so the two masks are cached at the model level and then re-used across layers.
  • Similar to the FA2 path, it works directly on the unpadded sequences
  • Re-uses the existing ModernBertRotaryEmbedding to avoid requiring FA2.

Note:
The current version requires one of the latest torch nightlies (e.g 2.6.0.dev20241112)
Currently transformers does not allow compiling the flex_attention function IIUC

@staghado staghado mentioned this pull request Dec 27, 2024
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