Fix the rushed out multi-query kernel

[zhuohan123]

1. Fix the correctness issue in the current FlashAttention-copy-based kernel. Make sure we call the FlashAttention kernel correctly. Evaluate the performance of this kernel.
2. Reduce the memory usage of the current kernel by limiting the buffer size and calling the kernel multiple times.

[tmm1]

current FlashAttention-copy-based kernel

I believe this is referring to #4, however as of #70 flash-attn is no longer used.

[hmellor]

Closing based on @tmm1's comment about flash-attn no longer being used.