Adds a print_applied method to QNodes

[josh146]

Context: Currently, it is hard to debug QNodes to ensure that they are executing the correct quantum circuit, for instance, if they include classical branching/logic within the QNode.

Description of the Change: Adds a print_applied method to QNodes, that prints the operation and observable queue as last constructed.

Example:

import pennylane as qml
from pennylane.templates.embeddings import AngleEmbedding
from pennylane.templates.layers import StronglyEntanglingLayers
from pennylane.init import strong_ent_layer_uniform

dev = qml.device('default.qubit', wires=2)

@qml.qnode(dev, interface="torch")
def circuit(weights, x=None):
    AngleEmbedding(x, [0,1])
    StronglyEntanglingLayers(weights=weights, wires=[0,1])
    return qml.expval(qml.PauliZ(0))

init_weights = strong_ent_layer_uniform(n_wires=2)

circuit(init_weights, x=[1., 2.])
circuit.print_applied()

Output:

Operations
==========
RX(1.0, wires=[0])
RX(2.0, wires=[1])
Rot(5.2934758701132685, 2.6339939891643005, 6.268309774410444, wires=[0])
Rot(2.599559228882876, 2.0359675113467888, 0.7480448828174393, wires=[1])
CNOT(wires=[0, 1])
CNOT(wires=[1, 0])

Observables
===========
expval(PauliZ(wires=[0]))

Benefits: Easier to debug complicated QNodes.

Possible Drawbacks: Since the queue is not constructed until evaluation, using print_applied() will prior to first evaluation will emit a warning message and no information.

Related GitHub Issues: n/a (was mentioned on the discussion forum).

[codecov]

Codecov Report

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The diff coverage is 100%.

@@            Coverage Diff            @@
##             master     #378   +/-   ##
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  Coverage          ?   99.58%           
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  Files             ?       32           
  Lines             ?     2404           
  Branches          ?        0           
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  Hits              ?     2394           
  Misses            ?       10           
  Partials          ?        0

Impacted Files	Coverage Δ
pennylane/interfaces/tf.py	100% <100%> (ø)
pennylane/qnode.py	100% <100%> (ø)
pennylane/interfaces/torch.py	100% <100%> (ø)
pennylane/decorator.py	100% <100%> (ø)

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[johannesjmeyer]

Just out of curiosity, are the names of self.queue and self.ev changed in the new QNode? I think they are very suboptimal.

[josh146]

They should be changed, yes; feel free to double check in #354

[johannesjmeyer]

Thanks for this neat contribution! It looks good, I have no objections.

[johannesjmeyer]

One could think of having this in a separate test, but I think two assert statements in one test are fine.

[josh146]

Trying to keep it contained for now, as it will need to be moved to the new QNode 😆