[FlashMatrix]: avoid computation in ifelse

[zheng-da]

    X1 = -fp.log(1 + fp.exp(-X))
    X2 = X - fp.log(1 + fp.exp(X))
    return fp.where(X > 0, X1, X2)

The code above has redundant computation. log and exp can be expensive. We should avoid unnecessary computation here.