Should not override `.controlled` for custom controlled implementations

[anurudhp]

Bloq authors should always override get_ctrl_system and not controlled. I think only the old cirq-ft bloqs ( QubitizationWalkOperator and related) override controlled directly, which must be upgraded.

from qualtran import Bloq
from qualtran.bloqs.for_testing.random_select_and_prepare import RandomPrepareOracle
from qualtran.bloqs.reflection_using_prepare import ReflectionUsingPrepare
from qualtran.drawing import show_bloq

prepare = RandomPrepareOracle.create(3, random_state=42)
R = ReflectionUsingPrepare(prepare)

class Wrapped(Bloq):
    @property
    def signature(self) -> 'Signature':
        return R.signature

    def build_composite_bloq(self, bb: 'BloqBuilder', **soqs: 'SoquetT') -> dict[str, 'SoquetT']:
        return bb.add_d(R, **soqs)

cb_direct = R.controlled().decompose_bloq()
cb_wrapped = Wrapped().controlled().decompose_bloq().flatten_once(lambda _: True)

show_bloq(cb_direct)

show_bloq(cb_wrapped)

More detailed notebook with above code: https://github.com/anurudhp/Qualtran/blob/2024-04-issue-controlled/controlled_issue.ipynb

[anurudhp]

We should also remove this: https://github.com/quantumlib/Qualtran/blob/c78dd0b5639b7fb4f471b47d59ff13042ca044c4/qualtran/_infra/gate_with_registers.py#L446-L448

[anurudhp]

related issue #649