[Bug]: FixedFrequencyTransmonPair class doesn't have a coupler attribute

[bsafvatiQM]

Description

I am trying to define a quam state with fixed frequency transmons with parametrically driven tunable couplers between them. However I see that if I define my qubits and qubit pairs to be FixedFrequencyTransmons, then there is no TunableCoupler class to add to FixedFrequencyTransmonPair. We should add this attribute as a default of this class.

Steps to Reproduce

1. Create quam state with FixedFrequencyTransmonPair and see that TunableCoupler is not an attribute by default.
2. Can also look at quam_builder\architecture\superconducting\qubit_pair\fixed_frequency_transmon_pair.py

Expected Behavior

I expect to be able to make a tunable coupler between two fixed frequency transmons.

Actual Behavior

Instead, I have to customize the default FixedFrequencyTransmonPair class to have coupler as an attribute.

Code Sample

@quam_dataclass
class FixedTransmonPairWithCoupler(FixedFrequencyTransmonPair):
    """FixedFrequencyTransmonPair extended with an optional tunable coupler.

    The upstream class only declares coupler on FluxTunableTransmonPair.
    This local subclass adds it so fixed-frequency pairs with coupler flux
    lines serialize and deserialize correctly.
    """

    coupler: Optional[TunableCoupler] = None

Error Message / Traceback

quam-builder Version

0.1.2

Python Version

3.11.13

Operating System

Windows 11

Additional Context

No response

Checklist

• I have searched existing issues to ensure this is not a duplicate
• I have provided all the information requested above