Fix FA2 for models with HybridCache #35681
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| if attention_mask.shape[-1] <= 1: # when decoding | ||
| attention_mask = attention_mask[:, :, :, -self.sliding_window :] |
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this is super important ! Why is it removed?
I know it is counter intuitive, but _flash_attention_forward takes the attention mask to pad / unpad the input itds.
Thus you need the slicing otherwise this operation fails, see the blame !
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Indeed, I was too fast on this one, the HybridCache behaves slightly differently than I remembered. There was still an issue in the slicing during prefill for FA2 though!
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What does this PR do?
As per the title. Models with HybridCache need to correctly slice the key/value states when using FA2 as inputs needs to be unpadded on the right as well (and the mask has shape [bs, seq_len]). It is currently broken and leads to garbage generation when using padding. This fixes it.
Also do not slice the mask during prefill if longer than the sliding window (I added a test for this case).