I was going through a paper in which protocol said that the party computes modulo 'd' sum of other party secrets directly. It was not mentioned that the computation is classical. Can I assume that it was done classically? Am I wrong?
Yes, assume it's classical.
That is, unless the protocol requires you to start computing it on superpositions. In that case, you'd have to perform the calculation on a quantum computer, although you can still use the reversible version of the classical algorithm on that quantum computer (because quantum computers can implement any classical computation, you just have to embed it in a reversible form first).