[QIM-5] Add mutual information function

[eddddddy]

Context:
Part of the ongoing effort to support quantum information computations in PennyLane.

Description of the Change:
Add support for mutual information. Three separate functions are implemented. One is in qml.math and takes as input a state vector or a density matrix:

>>> state = np.array([1, 0, 0, 1])
>>> qml.math.to_mutual_info(state, indices0=[0], indices1=[1])
1.3862943611198906

The second is in qml.qinfo and is a QNode transform:

dev = qml.device("default.qubit", wires=2)

@qml.qnode(dev)
def circuit(x):
    qml.RY(x, wires=[0])
    qml.CNOT(wires=[0, 1])
    return qml.state()

>>> x = np.array(0.5)
>>> mutual_info_qnode = qml.qinfo.mutual_info_transform(circuit, [0], [1])
>>> mutual_info_qnode(x)
0.46056096825956955

The last is a measurement process that returns the mutual information of a circuit:

dev = qml.device("default.qubit", wires=2)

@qml.qnode(dev)
def circuit(x):
    qml.RY(x, wires=[0])
    qml.CNOT(wires=[0, 1])
    return qml.mutual_info(wires0=[0], wires1=[1])

>>> x = np.array(0.5)
>>> circuit(x)
tensor(0.46056097, requires_grad=True)

Benefits:

PennyLane now supports computation of mutual information

Possible Drawbacks:

Related GitHub Issues: