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integrate xformers memory_efficient_attention, could be beneficial if your device's architecture is less than <sm_80
The efficiency between xformers.memory_efficient_attention and flash_attn in >sm_80 are almost the same.
But xformer does not expand the kv automatically, we need to do it manually, and the martix operation make this implemention much slower.
So the open logic is, try using flash_attn first and then try using xformer and then using Torch matmul attention.