unittest: Add head dim 256 test cases and mark as xfail#1999

yzh119 merged 3 commits intoflashinfer-ai:mainflashinfer-ai/flashinfer:mainfrom

bkryu:trtllm_gen_decode_head_256bkryu/flashinfer:trtllm_gen_decode_head_256Copy head branch name to clipboard