Lazy loading of Quantumscript properties

[EmilianoG-byte]

Context:
Eager initialization of parameters inside the constructor of QuantumScript induces a high classical overhead.

Description of the Change:
The following attributes of QuantumScript are no longer initialized in the constructor, but only when needed:

• par_info
• wires, num_wires
• obs_sharing_wires, obs_sharing_wires_id

par_info, obs_sharing_wires, and obs_sharing_wires_id are now public attributes as they were used in several files outside the QuantumScript class. Examples of this behaviour are:

• pennylane/_qubit_device.py
• pennylane/pennylane/circuit_graph.py
• pennylane/pennylane/fourier/qnode_spectrum.py
• pennylane/pennylane/gradients/adjoint_metric_tensor.py
• pennylane/pennylane/gradients/gradient_transform.py
• pennylane/pennylane/transforms/tape_expand.py

The most expensive attributes: par_info and wires are now a @cached_property to avoid calculating them every time they are needed.

When used inside the QuantumTape context, the _update() method is called to invalidate the cached value and allow for recalculation.

Benefits:
Reduce in the classical overhead of creating a QuantumScript. For instance, for the following code:

n_wires = 5
n_layers = 50

dev = qml.device('null.qubit')

@qml.qnode(dev, diff_method="parameter-shift")
def circuit(params):
    qml.StronglyEntanglingLayers(params, wires=range(n_wires))
    return qml.expval(qml.PauliZ(n_wires-1))

shape = qml.StronglyEntanglingLayers.shape(n_wires=n_wires, n_layers=n_layers)
rng = np.random.default_rng(seed=42)
params = np.array(rng.random(shape))
qml.grad(circuit)(params)

The QuantumScript creation time decreases from 62.51% of the whole time, to 41.92%.

Related Shortcut Stories:
[sc-45973]

[github-actions]

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Please edit doc/releases/changelog-dev.md with:

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[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.67%. Comparing base (58668d7) to head (563c81c).
Report is 4 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5696      +/-   ##
==========================================
- Coverage   99.68%   99.67%   -0.01%     
==========================================
  Files         416      416              
  Lines       39049    38808     -241     
==========================================
- Hits        38925    38683     -242     
- Misses        124      125       +1     

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[mudit2812]

Looks great! Just one comment, otherwise ready to approve.

[albi3ro]

👍 Looks good to me 🎉

[mudit2812]

Looks good, one very tiny suggestion and then I'll approve.