Deserializing a SingleQubitQPDGate results in a generic Instruction

[garrison]

With #417 solved by #442, it is now possible to serialize a SingleQubitQPDGate. However, when it is unserialized by qpy, the result is of type Instruction rather than SingleQubitQPDGate. It should be a SingleQubitQPDGate after a serialization/deserialization round trip.

Here's a minimum working example:

import io
from qiskit import QuantumCircuit, qpy

from circuit_knitting.cutting import partition_problem

qc = QuantumCircuit(4)
qc.cx(1, 2)

partitioned_problem = partition_problem(circuit=qc, partition_labels="AABB")
qc = partitioned_problem.subcircuits["A"]

f = io.BytesIO()
qpy.dump(qc, f)
qc2 = qpy.load(io.BytesIO(f.getvalue()))[0]

print(type(qc.data[0].operation))
print(type(qc2.data[0].operation))

gives the following output

<class 'circuit_knitting.cutting.qpd.instructions.qpd_gate.SingleQubitQPDGate'>
<class 'qiskit.circuit.instruction.Instruction'>

This, of course, has some implications: the resulting instructions don't actually behave like SingleQubitQPDGates, even though they have the correct name.

We should find a way to fix this.