run_autograph is now idempotent

[andrijapau]

Context:

When support for control flow with autograph (through the QNode execution pipeline) was introduced in #6837 a minor bug was discovered where if you try to apply autograph to an already autograph'd function, an ambiguous error would show up,

qml.capture.enable()
from pennylane.capture import run_autograph

dev = qml.device("default.qubit", wires=1)

@qml.qnode(qml.device("default.qubit", wires=1), autograph=True)
def circ(x: float):
    qml.RY(x, wires=0)
    return qml.expval(qml.PauliZ(0))

>>> ag_circ = run_autograph(circ)
>>> ag_ag_circ = run_autograph(ag_circ)
ValueError: closure mismatch, requested ('ag__',), but source function had ()

There are a few routes to solve this issue,

1. Raise a more helpful error
2. Throw a warning and proceed with converted function (Catalyst does this)
3. Silently proceed with converted function

Description of the Change:

Option (2) was preferred and so if an already converted function is detected (by the presence of the ag_uncoverted attribute), we throw an AutoGraphWarning and just return early. Now we have the behaviour,

qml.capture.enable()
from pennylane.capture import run_autograph

dev = qml.device("default.qubit", wires=1)

@qml.qnode(qml.device("default.qubit", wires=1), autograph=True)
def circ(x: float):
    qml.RY(x, wires=0)
    return qml.expval(qml.PauliZ(0))

ag_circ = run_autograph(circ)
>>> ag_ag_circ = run_autograph(ag_circ)
AutoGraphWarning: AutoGraph will not transform the function <function ...> as it has already been transformed.
>>> ag_ag_circ.func is ag_circ.func
True
>>> ag_ag_circ(-1)
Array(0.5403023, dtype=float32)

Benefits: Enables autograph specific tests to run with the default autograph=True argument through the QNode.

Possible Drawbacks:

Suppressed the warnings in the autograph test folder. This means that a future test that emits this warning could be missed if we don't pay attention to the WAE action that is run weekly.

Fixes [sc-83366]

[codecov]

Codecov Report

All modified and coverable lines are covered by tests ✅

Project coverage is 99.60%. Comparing base (64e226f) to head (55d488d).
Report is 3 commits behind head on master.

Additional details and impacted files

@@           Coverage Diff            @@
##           master    #7001    +/-   ##
========================================
  Coverage   99.60%   99.60%            
========================================
  Files         484      484            
  Lines       46141    46249   +108     
========================================
+ Hits        45957    46065   +108     
  Misses        184      184            

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[albi3ro]

👍

[lillian542]

LGTM! 🚀