Understand how to deal with zero-time gates in mitiq.ddd

[andreamari]

In some backends (e.g. IBM) there are some gates that have zero time duration, since they are applied in a virtual way. For example Rz gates have zero duration.

The currrent DDD technique in Mitiq, on the other hand, assumes that all gates take the same amount of time.

It would be great to solve this inconsistency, however, I don't have a straightforward solution in mind.

A non-optimal but perhaps interesting workaround could be to insert, after the full DDD process, an identity gate after each Rz gate in the final circuit. In this way we can force the execution to Rz gates to really last for a unit of time instead of zero.