Function build_eq_x_r_inner 

pub fn build_eq_x_r_inner<F>(
    r: &[F],
    cfg: &F::Config,
) -> Result<DenseMultilinearExtension<F::Inner>, ArithErrors>
where
    F: PrimeField,
    F::Inner: Zero,

This function build the eq(x, r) polynomial for any given r.

Evaluate eq(x,y) = \prod_i=1^num_var (x_i * y_i + (1-x_i)(1-y_i)) over r, which is eq(x,y) = \prod_i=1^num_var (x_i * r_i + (1-x_i)(1-r_i))