[Flash Attention] Block-Diagonal Masking Support

[WREQI]

not very efficient especially for consumer grade && mobile devices.

No dream = No life:

flash attention，block diagonal masking。

[laggui]

Flash attention is a work in progress, see #4325. You can actively follow the changes in cubek-attention if interested.

Block-diagonal masking is not currently supported in a way that exploits sparsity. Will mark as a feature request.