ggml-cuda: native bf16 flash attention for vec kernel (llama/20525)

ggml-cuda: native bf16 flash attention for vec kernel (llama/20525)

* ggml-cuda: native bf16 flash attention for vec and tile kernels

mma kernel still converts bf16 to fp16 before launch, native mma bf16 todo

* ggml-cuda: address code owner review feedback

reverted tile kernel changes to avoid larger refactor

* fix ci failures on turing and hip

* fix bf16 vec kernel compile on hip v_dot2 platforms

* add comments

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Co-authored-by: Johannes Gäßler <redacted>