Fix partition_problem's handling of unlabeled TwoQubitQPDGates

[garrison]

I noticed when experimenting with partition_problem that it blows up if you give it a circuit that contains an unlabeled TwoQubitQPDGate. In our workflows so far, all TwoQubitQPDGates are created within partition_problem itself, as the first thing it does is call partition_circuit_qubits, which creates the gates with the labels. However, it's not strictly necessary for the gates to have labels, and people might want to place some TwoQubitQPDGates manually in addition to the ones across the partition labels. (Also, in the future, we might want to make partition_labels optional, at which point it will just partition as well as it can using the TwoQubitQPDGates that have been placed.)

I double checked, and the added test indeed fails without the accompanying change.

[caleb-johnson]

awesome, nice catch 👍