Function assign_witnesses 

pub fn assign_witnesses<F: ScalarField>(
    threads: &[Context<F>],
    basic_gates: &[BasicGateConfig<F>],
    region: &mut Region<'_, F>,
    break_points: &ThreadBreakPoints,
)

Assigns all virtual threads to the physical columns in basic_gates according to a precomputed “computation graph” given by break_points. (break_points tells the assigner when to move to the next column.)

This function does not impose any constraints. It only assigns witnesses to advice columns, and should be called only during proof generation.

Assumptions

• All basic_gates are in the same phase.