In this PR I apply the same logic from #187 to the reduce collecive communication algorithm.

The logic is the same than for broadcast() but inverted.

1. If we are the receiver of the reduce, we expect all our local ranks to send their data for reduction. We also expect all remote leaders to send us their data.
2. If we are a local leader such that we are not co-located with the receiver rank: we (i) wait for all our local ranks to send us their data for reduction, and (ii) send our reduced data to the receiver of the reduce (1).
3. If we are neither the receiver rank nor a local leader, we send our data for reduction to either our local master or the receiver: if we are colocated with the receiver, we send to the receiver, and if not we send to our local master.

Note that this assumes all our reduce operations to be associative and commutative, which is an assumption we already make.