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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