FastTree returns incorrect results when multiple prefixes exist at level 0

[March-H]

Hi, thanks for the great work on this project!

I've encountered an issue with FastTree where it fails to return correct results when multiple distinct prefixes exist at the root level(level 0).

[INFO] Generating tree file: trees/nc_4,16_4096,256.txt
[INFO] Running benchmark.py -> outputs/nc_4,16_4096,256_qk_32_8.txt
flash attn time: 0.2109760046005249
fasttree attn time: 0.1045759990811348
flash attn time: 0.20908799767494202
fasttree attn time: 0.10408000648021698
fasttree attn Check failed!
fasttree 10 time: 0.31993600726127625
fasttree -10 time: 0.10478399693965912

[March-H]

it only work with same sys prompt in level 0