Fix sign direction.

Author: liuliu

examples/txt2img/main.swift

@@ -190,7 +190,7 @@ func uniPBhUpdate(
  let xt_ = Functional.add(
  left: x, right: m0, leftScalar: sigmat / sigmas0, rightScalar: -alphat * hPhi1)
  if let D1 = D1 {
- let xt = Functional.add(left: xt_, right: D1, leftScalar: 1, rightScalar: alphat * Bh * rhosP)
+ let xt = Functional.add(left: xt_, right: D1, leftScalar: 1, rightScalar: -alphat * Bh * rhosP)
  return xt
  } else {
  return xt_