Write an optimized flash_attn_stream_k_fixup kernel (llama/21159)

Write an optimized flash_attn_stream_k_fixup kernel (llama/21159)

* Write an optimized flash_attn_stream_k_fixup kernel

Write a specialized and more optimized kernel for cases where nblocks_stream_k is multiple of ntiles_dst.
Make nblocks_stream_k to multiple of ntiles_dst if nblocks_stream_k > 2 * ntiles_dst

* Use the new kernel only for nblocks_stream_k_raw > 4 * ntiles_dst to make sure we have enough concurrency on GPUs

* Address review comments

* Address review comments

* Revert variable names to original