Fix return of single measurement in qfunc transform.

[glassnotes]

Context: After applying a quantum function transform and constructing a QNode, the resultant QNode would return a 1-d array of measurement results even when there was only a single measurement.

Description of the Change: The qfunc transform decorator now checks the length of the list of measurements, and returns a list of results only when there are multiple measurements.

Benefits: Transformed QNodes that return a 0-d tensor now still return a 0-d tensor.

Possible Drawbacks: None

Related GitHub Issues: None

[github-actions]

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[codecov]

Codecov Report

Merging #1434 (284e65f) into master (a6a82ee) will increase coverage by 0.00%.
The diff coverage is 100.00%.

@@           Coverage Diff           @@
##           master    #1434   +/-   ##
=======================================
  Coverage   98.23%   98.23%           
=======================================
  Files         160      160           
  Lines       11964    11966    +2     
=======================================
+ Hits        11753    11755    +2     
  Misses        211      211           

Impacted Files	Coverage Δ
pennylane/transforms/qfunc_transforms.py	100.00% <100.00%> (ø)

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[josh146]

Amazing, thanks @glassnotes!