Split QFI into QFI and QGT

[Cryoris]

What should we add?

The current QFI implementation doesn't properly reflect that the QFI is purely real, as the code allows to take also imaginary parts of it. Instead we should have a class for the quantum geometric tensor (QGT) that allows to take real, imaginary and real+imaginary parts and build the QFI on it, using QFI = 4 * Re(QGT). In summary:

class QGT:
    def __init__(self, derivative_type=DerivativeType.COMPLEX):
        ...

class LinCombQGT:
    ...

class LinCombQFI:
    def __init__(self):
        self.qgt = LinCombQGT(derivative_type=DerivativeType.REAL)
        
    def run(self, ...):
        return 4 * self.qgt.run(...) 

[Cryoris]

@a-matsuo

[Cryoris]

As discussed, we're adding this to the 0.23 milestone again 🙂