Function flatten 

pub fn flatten(
    lib: &Library,
    cell: CellId,
    max_depth: usize,
    new_name: impl Into<CellName>,
) -> CellBuilder

Flatten a cell’s instance hierarchy max_depth levels deep into a single cell. max_depth = 0 is a no-op (no children flattened); max_depth = usize::MAX flattens everything.