Fix params.seqlen_k reference in the splitkv kernel to binfo.actual_seqlen_k (#18)

Author: cakeng

csrc/flash_attn/src/flash_fwd_kernel.h

@@ -531,7 +531,7 @@ inline __device__ void compute_attn_1rowblock_splitkv(const Params &params, cons
     // if (threadIdx.x == 0 && blockIdx.y == 1 && blockIdx.z == 0) { printf("params.knew_ptr = %p, seqlen_k_cache + seqlen_knew = %d\n", params.knew_ptr, binfo.seqlen_k_cache + (params.knew_ptr == nullptr ? 0 : params.seqlen_knew)); }
     if (m_block * kBlockM >= binfo.actual_seqlen_q) return;
 
-    const int n_blocks_per_split = ((params.seqlen_k + kBlockN - 1) / kBlockN + num_n_splits - 1) / num_n_splits;
+    const int n_blocks_per_split = ((binfo.actual_seqlen_k + kBlockN - 1) / kBlockN + num_n_splits - 1) / num_n_splits;
     const int n_block_min = !Is_local
         ? n_split_idx * n_blocks_per_split
         : std::max(n_split_idx * n_blocks_per_split, (m_block * kBlockM + binfo.actual_seqlen_k - binfo.actual_seqlen_q - params.window_size_left) / kBlockN);