Do not cast state to complex128

[vincentmr]

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---

Context:
The LQ new device API does not preserve the dtype of measurement results,

import pennylane as qml
import numpy as np
dev = qml.device("lightning.qubit", wires=2, c_dtype=np.complex64)

@qml.qnode(dev)
def circ():
    return qml.state()

>>> circ().dtype
complex128

The issue comes from measurementprocess.process_state as this method changes the specified dtype to the default complex128.

Description of the Change:
Modify StateMP and DensityMatrixMP avoiding explicitly casting to complex128, relying on the various frameworks casting rules, by adding 0.0j. This does not work in TensorFlow for which the current behaviour is preserved.

Benefits:

Possible Drawbacks:

Related GitHub Issues:
[sc-60855]

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.68%. Comparing base (a8bba50) to head (14bf29f).
Report is 5 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5547      +/-   ##
==========================================
- Coverage   99.69%   99.68%   -0.01%     
==========================================
  Files         410      410              
  Lines       38230    37943     -287     
==========================================
- Hits        38113    37825     -288     
- Misses        117      118       +1     

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[mudit2812]

Looks good! One open question, otherwise ready to approve.

[maliasadi]

🙌