Move to specify shots on the QNode

[albi3ro]

We are moving to define shots as part of what we execute, not necessarily how we execute it.

We already:

• Allow shots to be overridden on each call in the QNode
• Allow override_shots to be provided to qml.execute
• Allow shots to be specified in the QuantumTape
• Purely take shots from the QuantumTape in the new device API.

These changes will allow us to have better shot distribution within batches, since each tape in a batch will be able to have a separate shot number.

This will also help PennyLane become more functional, since we will not be needing to constantly mutate the device in order to support per-call dynamic shots.

Since these warnings are inside the QNode and the qml.device constructor, no changes are currently necessary for plugins other than updated tests and documentation.

Example behaviour for deprecation cycle

import pennylane as qml
import warnings
warnings.simplefilter("always")

Don't specify shots on device? No warning

qml.device('braket.local.qubit', wires=2)

<BraketLocalQubitDevice device (wires=2, shots=None) at 0x14001c400>

Manually specify shots on device? Warning

qml.device('braket.local.qubit', wires=2, shots=None)

/Users/christina/Prog/pennylane/pennylane/__init__.py:309: UserWarning: In v0.33, the shots will always be determined by the QNode. Please specify shots there.
  warnings.warn("In v0.33, the shots will always be determined by the QNode. Please specify shots there.", UserWarning)
<BraketLocalQubitDevice device (wires=2, shots=None) at 0x17fbd8a00>

Don't specify shots on either device or qnode? No warning

dev = qml.device('braket.local.qubit', wires=2)

@qml.qnode(dev)
def circuit():
    return qml.expval(qml.PauliX(0))

Specify shots on device but not qnode?

warning, and make qnode shots equal to device shots

dev = qml.device('braket.local.qubit', wires=2, shots=10)

/Users/christina/Prog/pennylane/pennylane/__init__.py:309: UserWarning: In v0.33, the shots will always be determined by the QNode. Please specify shots there.
  warnings.warn("In v0.33, the shots will always be determined by the QNode. Please specify shots there.", UserWarning)

@qml.qnode(dev)
def circuit():
    return qml.expval(qml.PauliX(0))

circuit(), circuit.default_shots

/Users/christina/Prog/pennylane/pennylane/qnode.py:448: UserWarning: Shots should now be specified on the qnode instead of on the device.Using shots from the device. QNode specified shots will be used in v0.33.
  warnings.warn(

(tensor(0.2, requires_grad=True),
 Shots(total_shots=10, shot_vector=(ShotCopies(10 shots x 1),)))

Shots not none on qnode? no warning when creating the qnode, but there is a warning when putting on them on the device still.

dev = qml.device('lightning.qubit', wires=2, shots=1000)

/Users/christina/Prog/pennylane/pennylane/__init__.py:309: UserWarning: In v0.33, the shots will always be determined by the QNode. Please specify shots there.
  warnings.warn("In v0.33, the shots will always be determined by the QNode. Please specify shots there.", UserWarning)

@qml.qnode(dev, shots=2)
def circuit():
    return qml.sample(wires=0)

circuit()

tensor([0, 0], requires_grad=True)

[github-actions]

Hello. You may have forgotten to update the changelog!
Please edit doc/releases/changelog-dev.md with:

• A one-to-two sentence description of the change. You may include a small working example for new features.
• A link back to this PR.
• Your name (or GitHub username) in the contributors section.

[albi3ro]

[sc-41930]

[trbromley]

Thanks @albi3ro , I left some wording suggestions.

[trbromley]

Could we say "Please specify shots either at QNode instantiation or when calling your QNode."

[trbromley]

And maybe "You have provided a shots argument to a device. In v0.33, ..."
Just so it's immediately clear to users what they've done wrong.

[trbromley]

Suggested change

	"Shots should now be specified on the qnode instead of on the device."
	"The shots value you provided to the device does not match the value provided to the QNode."

[trbromley]

Suggested change

	"Using shots from the device. QNode specified shots will be used in v0.33."
	"Using shots from the device. QNode specified shots will be used in v0.33 "
	"and shots will no longer be permitted as a device argument."

[albi3ro]

Closed in favor of #4388