[BUG] Can't initialize ParticleConservingU1/2 with default init_state

[dwierichs]

Expected behavior

ParticleConservingU1 and ParticleConservingU2 can be initialized with their default kwargs.

Actual behavior

They error out with init_state=None (the default setting)

Additional information

While it may be intended that users should provide an init_state, it is not good practice to have keyword arguments with invalid defaults. None should be interpreted as a valid init_state, like tuple(0 for _ in wires) or so.
If init_state should be provided by users always, init_state should be a positional arg, not a kwarg.

Source code

qml.ParticleConservingU1(np.ones((3, 1, 2)), [2, 3])
# qml.ParticleConservingU2(np.ones((3, 3)), [2, 3]) # U2 has the same problem

Tracebacks

---------------------------------------------------------------------------
TypeError                                 Traceback (most recent call last)
Cell In[6], line 2
      1 # qml.GateFabric(np.ones((3, 1, 2)), [0, 1, 1, 0], wires=[2, 3, 0, 1])
----> 2 qml.ParticleConservingU1(np.ones((3, 1, 2)), [2, 3])

File ~/repos/pennylane/pennylane/capture/meta_type.py:151, in PLXPRMeta.__call__(cls, *args, **kwargs)
    145 def __call__(cls, *args, **kwargs):
    146     # this method is called everytime we want to create an instance of the class.
    147     # default behavior uses __new__ then __init__
    148     # when tracing is enabled, we want to
    150     if not plxpr_enabled():
--> 151         return type.__call__(cls, *args, **kwargs)
    152     # use bind to construct the class if we want class construction to add it to the jaxpr
    153     return cls._primitive_bind_call(*args, **kwargs)

File ~/repos/pennylane/pennylane/templates/layers/particle_conserving_u1.py:269, in ParticleConservingU1.__init__(self, weights, wires, init_state, id)
    264 if shape[2] != 2:
    265     raise ValueError(
    266         f"Weights tensor must have third dimension of length 2; got {shape[2]}"
    267     )
--> 269 self._hyperparameters = {"init_state": tuple(init_state)}
    271 super().__init__(weights, wires=wires, id=id)

TypeError: 'NoneType' object is not iterable

System information

pl dev

Existing GitHub issues

• I have searched existing GitHub issues to make sure the issue does not already exist.