Fix qml.prod when qfunc contains a single operator

[albi3ro]

Context:

Found internally with the script

@qml.prod
def my_gate():
  qml.Hadamard(wires = 0)
  #qml.Hadamard(wires = 1)

dev = qml.device("default.qubit")

U = my_gate()

@qml.qnode(dev)
def circuit():

  my_gate()

  qml.ctrl(U, control = 2)

  return qml.probs(wires = [0,1,2])

qml.draw_mpl(circuit, wire_order = [0,1,2])()

Basically when a qfunc only has a single operator, it doesn't get turned into a Prod class, so it isn't re-queued.

Description of the Change:

Adds special logic for processing a qfunc with a single operator.

Benefits:

Possible Drawbacks:

Related GitHub Issues:

[sc-56341]

[astralcai]

Looks good!

[codecov-commenter]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (c1a813d) 99.68% compared to head (38a0ac1) 99.68%.
Report is 1 commits behind head on master.

Additional details and impacted files

@@            Coverage Diff             @@
##           master    #5170      +/-   ##
==========================================
- Coverage   99.68%   99.68%   -0.01%     
==========================================
  Files         394      394              
  Lines       36160    35892     -268     
==========================================
- Hits        36047    35778     -269     
- Misses        113      114       +1     

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[albi3ro]

But the difference is one uncovered break statement in merge_rotations, which this PR certainly didn't change. So I'm overriding.