Function gen_projected_data 

pub fn gen_projected_data(
    batch_size: usize,
    batch_num: usize,
    expr: BoxedExpression,
) -> Vec<DataChunk>

Generate batch_num data chunks with type Int64, each data chunk has cardinality of batch_size. Then project each data chunk with expr.

NOTE: For convenience, here we only use data type Int64.