Qubit specific extension to qibo noise model

[jykhoo1987]

The current composite NoiseModel is limited to the same 'depolarizing error' for one- and two-qubit gates across all qubits. It would be good to have the ability to input different values of for different qubits, which is likely the case as well in hardware. The ‘excited population’ should also have this flexibility as well.

@rahula06 @shangtai

[renatomello]

That's already allowed, because the qubits variable from the NoiseModel.add method allows to specify qubits using integers and tuples. See https://qibo.science/qibo/stable/api-reference/qibo.html#noise. So one already could specify noise per gate and per qubit.

EDIT: the documentation is not very clear that integers are allowed in the qubits parameter, so at least that very small fix should be done.