Support qml.sample() without specifying the observable

[king-p3nguin]

Issue #, if available:
Related to #98

Description of changes:

• qml.sample() can now be used without specifying the observable.

Example:

import pennylane as qml
from pennylane import numpy as np

dev_managed_sim = qml.device(
    "braket.aws.qubit",
    device_arn="arn:aws:braket:::device/quantum-simulator/amazon/sv1",
    wires=2,
    shots=10,
)

@qml.qnode(dev_managed_sim)
def circuit(a):
    qml.Hadamard(wires=0)
    qml.CNOT(wires=[0, 1])
    qml.RX(a, wires=1)
    # return qml.sample(qml.PauliZ(0))
    # return qml.sample()
    return qml.sample(wires=[0])


# x = np.array(0.543)
x = np.array([0.543, 0.3])
print(circuit(x))

output:

[[0 1 0 1 1 0 0 1 1 1]
 [1 0 1 0 1 0 0 1 1 0]]

Testing done:

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Codecov Report

Attention: Patch coverage is 33.33333% with 10 lines in your changes missing coverage. Please review.

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##              main     #266      +/-   ##
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- Coverage   100.00%   99.18%   -0.82%     
===========================================
  Files            7        7              
  Lines         1211     1223      +12     
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===========================================
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- Misses           0        8       +8     
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[rmshaffer]

This change is an interesting improvement, but I don't think it addresses the issue #179.

The idea is to support a vector for the shots argument, like: https://docs.pennylane.ai/en/stable/code/api/pennylane.devices.default_qubit.DefaultQubit.html?highlight=shot_vector#pennylane.devices.default_qubit.DefaultQubit.shot_vector

The vector would be able to indicate a different number of shots to execute each circuit, rather than having every circuit executed the same number of shots.,

[rmshaffer]

instead of embedding this logic here, would it be possible to have _translate_observable return both an observable and targets, and have it fall back to these defaults if observable is none and/or targets is none? Might be slightly cleaner (and then maybe wouldn't require the C901 suppression above either, since the logic would be outside this function).

[king-p3nguin]

I am not sure about that. The target cannot be specified if an observable is specified, so the branching will be based on whether the observable is None. Therefore, I believe that the overall logic will remain the same, and it will only be divided into more functions.

[rmshaffer]

same question here, would it be possible to embed this logic inside translate_result_type somehow?

[king-p3nguin]

Sorry for the misunderstanding. I do not think I will be able to provide a solution to issue #179 before the end of the unitary hack, so I will change the goal of the pull request to support qml.sample() without specifying the observable.