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Fix precision in Metal fused attention - #3119
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In our vector and NAX attention we keep the scale factor in fp32. But in the non-NAX fused attention it gets downcast to bf16 which is made worse by the fact that it is multiplied by another scale as well.
It seems to have a real impact on model quality in some cases: ml-explore/mlx-lm#868 (comment)
In terms of performance I think the regression is acceptable as it's less than 1% (at most 0.5%) for all the cases I tried
And in terms of accuracy the difference between the fused attention in bf16 and fp32 with a scaling factor is noticeably lower:
The maximum absolute difference goes down by a factor of 6. For some random inputs with a typical scaling factor based on the head dimension:
Pre: 0.00585938
Post: 0.000976562