[QIM-3] Add function density_matrix_transform

[rmoyard]

Quantum Information module PR # 3
Context:

After the creation of density matrix functions for state vectors #2554 and matrices #2569 , we add a QNode transform that is accepting QNodes returning state.

Description of the Change:

We add the QNode returning states argument in the general function density_matrix_transform in the qinfo module.

@qml.qnode(dev)
def circuit(x):
    qml.RZ(x, wires=0)
    return qml.state()

density_matrix = qml.qinfo.density_matrix_transform(circuit, wires=[0])(0)

Benefits:

The density_matrix_transform function is accepting QNodes returning state. It is available across the whole codebase.