Fix a bug related to causal mask

[rchardx]

For example, case (qo_len=2, kv_len=3) should have a causal mask as (kv_idx >= kv_len + q_idx - qo_len):

\begin{pmatrix}
0 & -\infty & -\infty\\ 
0 & 0 & -\infty
\end{pmatrix}

The causal mask by include/flashinfer/attention/prefill.cuh and src/cpu_reference.h (kv_idx > kv_len + q_idx - qo_len):

\begin{pmatrix}
0 & 0 & -\infty\\ 
0 & 0 & 0
\end{pmatrix}