cirq.unitary does not work for controlled QubitizationWalkOperator operator

[anurudhp]

example code:

import cirq
from qualtran.bloqs.for_testing.qubitization_walk_test import get_uniform_pauli_qubitized_walk

_, walk = get_uniform_pauli_qubitized_walk(4)
bloq = walk.controlled()

cbloq = bloq.as_composite_bloq()

sim = cirq.Simulator()
_ = sim.simulate(cbloq.to_cirq_circuit())

raises

TypeError: Simulator doesn't support Controlled(subbloq=QubitizationWalkOperator(block_encoding=SelectBlockEncoding(select=SelectPauliLCU(selection_bitsize=2, target_bitsize=4, select_unitaries=(cirq.DensePauliString('XYII', coefficient=(1+0j)), cirq.DensePauliString('IYZI', coefficient=(1+0j)), cirq.DensePauliString('IIZX', coefficient=(1+0j)), cirq.DensePauliString('YIIX', coefficient=(1+0j))), control_val=None), prepare=PrepareUniformSuperpositionTest(n=4, cvs=(), qlambda=4.0))), ctrl_spec=CtrlSpec(qdtypes=(QBit(),), cvs=(array(1),))).on(cirq.NamedQubit('ctrl'), cirq.NamedQubit('selection0'), cirq.NamedQubit('selection1'), cirq.NamedQubit('target0'), cirq.NamedQubit('target1'), cirq.NamedQubit('target2'), cirq.NamedQubit('target3'))

But decomposing once results in a simulable circuit:

_ = sim.simulate(cbloq.flatten_once().to_cirq_circuit())

I suspect it is some interop issue b/w cirq and GateWithRegisters. Could be related to #1488