Successive use of same UCCDefault instance adds more passes

[bachase]

Describe the bug
Each call to run on the same instance of UCCDefault1 adds additional passes:

ucc/ucc/transpilers/ucc_defaults.py

Lines 120 to 124 in 1c1b6f1

	self.add_map_passes(coupling_list)
	self.pass_manager.append(
	BasisTranslator(sel, target_basis=self.target_basis)
	)
	out_circuits = self.pass_manager.run(circuits)

To Reproduce
Run the following code

from ucc.transpilers.ucc_defaults import UCCDefault1
from qiskit import QuantumCircuit as QiskitCircuit

ucc_compiler = UCCDefault1()
circuit = QiskitCircuit(2)
circuit.h(0)
circuit.cx(0, 1)

for run in range(5):
    print(f"{run}: Num passes {len(ucc_compiler.pass_manager._tasks)}")
    custom_compiled_circuit = ucc_compiler.run(circuit)

gives output

0: Num passes 8
1: Num passes 9
2: Num passes 10
3: Num passes 11
4: Num passes 12

Expected behavior
I would expect the number of passes to remain fixed between runs, so an output of

0: Num passes 8
1: Num passes 8
2: Num passes 8
3: Num passes 8
4: Num passes 8

[bachase]

@jordandsullivan and @Misty-W -- I stumbled on this while trying to understand more about the set of default passes. I'm not sure if its actually intended that the passes are modified here, but it seemed surprising on first review.

[jordandsullivan]

So because the intended user interface is ucc.compile(circuit) (which initializes a fresh instance of UCCDefault1() on every call):

ucc/ucc/compile.py

Lines 53 to 56 in 1c1b6f1

	compiled_circuit = UCCDefault1().run(
	qiskit_circuit,
	coupling_list=get_backend_connectivity(target_device),
	)

we didn't consider successive calls to UCCDefault1().run() as a potential workflow, but I suppose we could separate the pass manager object and initialize it when calling UCCDefault1().run() rather than initalizing it with UCCDefault1(), as currently happens:

ucc/ucc/transpilers/ucc_defaults.py

Lines 37 to 39 in 1c1b6f1

	class UCCDefault1:
	def __init__(self, local_iterations=1):
	self.pass_manager = PassManager()

[bachase]

I had stumbled on the non-ucc.compiler(circuit) path when following this part of the docs around a non-default pass.

Do we view UCCDefault1 as an equivalent of a PassManager, just with some specific defaults? If so, perhaps we could initialize all the passes in the initializer instead. This would require users to provide the coupling_list at the time its created.

If we did this way, then the approach linked in the docs for adding additional custom passes would work directly, and I think would match user expectation that an instance of UCCDefault1 starts with the default set of passes already.

[jordandsullivan]

Gotcha, yes I agree with that approach. I think it would be more intuitive for contributor-users.