fix: replace if/else statement with tl.where

[Sara-KS]

While running the flash attention benchmark in a python environment, I encountered the following error
is_mqa = hq != hk
off_h_k = off_h_q % hk if is_mqa else off_h_q
^
Triton does not support if expressions (ternary operators) with dynamic conditions, use if statements instead

Version information
ROCm: 5.7.3
python: 3.11
Triton: pytorch-triton-rocm: 2.2.0
System: MI250 (gfx90a)

Solution: Replaced the if/else statement with tl.where function. With this replacement, the benchmark runs without error message.