From the doc string:

Operations marked with this tag are presumed to have zero duration of their own, although they may have a non-zero duration if run in the same Moment as a non-virtual operation.

This is very confusing because noise certainly has "duration". Of course we only support "noise" in simulation (well on the quantum computers too haha).

Do we have an example of a noise composition that is useful? Right now noise composition feels dangerous since noise does not commute and the new model is highly dependent on this ordering.