Based on the documentation, MIG support was added in V2021.1.1. You also need to have the cgroups hook enabled. If you do that, PBS should automatically recognize the instances. For instance, if you were on a node with (4) A100s and MIG mode was not on, the cgroups hook would set ngpus=4. If you then put all four A100s into MIG mode and configured them to each have 7 instances, then the cgroups hook would report ngpus=28. Note that PBS does not differentiate the size of the instance. If you configured one A100 into 7 instances, one into two instances, one into 3 instances, and one into only one instance, the cgroups hook would report that as ngpus=13 and you could randomly get an instance that is 1/7th of the A100 or the full A100.
I hope that helps.