fix: improve parameter batch handling for Z-basis gates in default.mixed

[andrijapau]

Context:

See #8244.

Description of the Change:

Patches the logic so the correct expand shape is given to .reshape.

Fixes #8244

[sc-99115]

[codecov]

Codecov Report

✅ All modified and coverable lines are covered by tests.
✅ Project coverage is 99.39%. Comparing base (3600b4b) to head (ebf6f6e).
⚠️ Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff           @@
##           master    #8251   +/-   ##
=======================================
  Coverage   99.39%   99.39%           
=======================================
  Files         551      551           
  Lines       57214    57216    +2     
=======================================
+ Hits        56870    56872    +2     
  Misses        344      344           

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[JerryChen97]

Thanks a lot! Have we checked if this works for the user's circuit now:

import pennylane as qml

@qml.qnode(device=qml.device("default.mixed", wires=1))
def circuit(theta):
    qml.RZ(theta, wires=0)
    return qml.density_matrix(wires=[0])
circuit(np.array([1,2]))

[andrijapau]

Thanks a lot! Have we checked if this works for the user's circuit now:

import pennylane as qml

@qml.qnode(device=qml.device("default.mixed", wires=1))
def circuit(theta):
    qml.RZ(theta, wires=0)
    return qml.density_matrix(wires=[0])
circuit(np.array([1,2]))

Yup!