[Frontend] Fix AutoGraph not converting functions wrapped by 'adjoint', 'grad', etc.

[dime10]

The following example would fail before the fix:

 def inner(n):
    for i in range(n):
      qml.T(i)

  @qjit(autograph=True)
  @qml.qnode(qml.device("lightning.qubit", wires=5))
  def f(n: int):
      adjoint(inner)(n)
      return qml.state()

[sc-48511]

[erick-xanadu]

I am not an expert in autograph yet. The tests look good. I have noticed that some tests use the variable backend while others are hard coded to be lightning. I think it is best if we always use backend.

[dime10]

The tests look good. I have noticed that some tests use the variable backend while others are hard coded to be lightning. I think it is best if we always use backend.

I deliberately didn't use it here because I'm only testing frontend functionality. I just had to use a QNode to use adjoint, otherwise I wouldn't have. I agree that most tests should use backend, unless they are not testing execution functionality.

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Comparison is base (b729a19) 99.56% compared to head (2f79f1b) 99.56%.

Additional details and impacted files

@@           Coverage Diff           @@
##             main     #336   +/-   ##
=======================================
  Coverage   99.56%   99.56%           
=======================================
  Files          42       42           
  Lines        7404     7410    +6     
  Branches      432      433    +1     
=======================================
+ Hits         7372     7378    +6     
  Misses         16       16           
  Partials       16       16           

Files	Coverage Δ
frontend/catalyst/ag_primitives.py	97.92% <100.00%> (+0.06%)	⬆️

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