Function build_sequence 

pub fn build_sequence<T: Tokenizer + ?Sized>(
    tok: &T,
    state: &Value,
    q: &InternalQ,
    max_len: usize,
    head_max_len: usize,
    option_order: Option<&[usize]>,
    truncate_left: bool,
) -> Result<(Vec<u32>, Vec<usize>)>

Build one token sequence + option marker positions.

Format: [CLS] <type> question: <instructions> [SEP] [MASK] opt0 [MASK] opt1 ... [SEP] state [SEP]. Ported verbatim from build_sequence in laya/common.py.