Fixes a bug where the CV param-shift would error with more than one 2nd order observable

[josh146]

Context:

The CV parameter-shift rule will raise an exception if attempting to differentiate a QNode with more than one second order observable. For example, running the following script:

dev = qml.device("default.gaussian", wires=2)

@qml.qnode(dev)
def circuit(r):
    qml.Squeezing(r, 0, wires=0)
    qml.Squeezing(r, 0, wires=1)
    return [qml.expval(qml.NumberOperator(i)) for i in range(2)]

print(qml.jacobian(circuit)(0.5))

gives the error:

  File "/home/josh/xanadu/pennylane/pennylane/tape/cv_param_shift.py", line 375, in processing_fn
    grad[transformed_obs_idx] = res
ValueError: shape mismatch: value array of shape (2,) could not be broadcast to indexing result of shape (1,)

Description of the Change:

Fixes the bug; the above example now prints

[1.17520119 1.17520119]

Benefits: Bug fixed.

Possible Drawbacks: n/a

Related GitHub Issues: https://discuss.pennylane.ai/t/typeerror-while-using-qml-gradientdescentoptimizer/956/3

[josh146]

The previous implementation assumed that res was always a scalar. This is only the case if the QNode has only one observable.

[codecov]

Codecov Report

Merging #1197 (ea4b90f) into master (816885a) will not change coverage.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1197   +/-   ##
=======================================
  Coverage   98.12%   98.12%           
=======================================
  Files         145      145           
  Lines       10980    10980           
=======================================
  Hits        10774    10774           
  Misses        206      206           

Impacted Files	Coverage Δ
pennylane/tape/cv_param_shift.py	98.72% <100.00%> (ø)

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[trbromley]

Thanks @josh146!