Fix: use cu_chunk instead of cu_seqlens for flash_attn during corner case where top-k equals 1

[yibozhong]

This PR fixes an inconsistency in MoBA. When top-k > 1, the self attn is calculated using cu_seqlens_q=self_attn_cu_seqlen, where self_attn_cu_seqlen = cu_chunk. Meanwhile when top-k = 1, the self attn is calculated using cu_seqlens_q=cu_seqlens, which behaves the same as standard attention. Because the current block is always chosen, the corner case should use cu_chunk instead of cu_seqlens.