* Adding BF16 support for AVX2
PP performance is the same as fp16 (~153 t/s on Ryzen-5975WX),
but TG is quite a bit lower (3.65 t/s vs 4.72 t/s at 8 threads).
Why?
* Slightly faster fp16/bf16 gemv on AVX2
It still saturates at the same lower peformance for bf16
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Co-authored-by: Iwan Kawrakow <iwan.kawrakow@gmail.com>
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