Transforms shallow copy qnode

[albi3ro]

Closes #4731 [sc-48599]

Currently, transforms deep copy the qnode when they are applied. Unfortunately, this also deep copies the device.

Copying the device leads to issues like:

1. pickling errors for lightning.kokkos (see above issue)

2. Unnecessary allocation of large amounts of memory

3. No longer being able to use the original devices' tracker:

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

@qml.transforms.compile
@qml.qnode(dev)
def circuit():
    return qml.state()

with dev.tracker:
    circuit()
dev.tracker.totals

{}

This bugfix downgrades the deep copy of a qnode to a shallow copy that preserves the device.

To better separate the original from the copy, QNode now defines a custom __copy__ method that shallow copies the execute_kwargs, transform_program, and gradient_kwargs. These were the three fully mutable properties of the qnode.

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[rmoyard]

Just two comments, otherwise looks good! Thanks 💯

[timmysilv]

if it's just the device, what about a simpler copy, like:

device = self.device
self.device = None
qnode_copy = super().__copy__(self)
self.device = qnode_copy.device = device
return qnode_copy

[albi3ro]

if it's just the device, what about a simpler copy, like:

device = self.device
self.device = None
qnode_copy = super().__copy__(self)
self.device = qnode_copy.device = device
return qnode_copy

@timmysilv

Is this a proposal for QNode.__copy__, or for use in TransformDispatcher.default_qnode_transform?

super().__copy__ is not defined, so we can't use that.

[timmysilv]

it was for qnode.copy. is there no default copy behaviour if an object doesn't override the dunder? maybe not super(), but I assumed there was, and that we could access that. anyway, nvm if it's more of a pain

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[rmoyard]

Thanks it looks good 👍