center O_k
Created by: inailuig
Currently the on_the_fly SR code is using S = ⟨Ô†ΔÔ⟩ for the quantum geometric tensor.
This adds to option to use S = ⟨ΔÔ†ΔÔ⟩ with a centered wavefunction instead and makes it the default.
This might be better numerically, but comes at the cost of slightly more communication (one allreduce of the parameters) and one more forward pass with the slightly more complicated centered wavefunction.