Implement quantum state reconstruction (pure & mixed)
Created by: JasonZHM
This pull request implements the quantum state reconstruction driver which minimizes the KL divergence to approximate the target state.
Features:
- compatible with both pure and mixed ansatz
- implement a control variates method (stochastic variance reduced gradient, SVRG) to reduce the variance caused by mini-batch training
- support chunking when computing control variate expectation
- an example of reconstructing 1d Ising chain ground state
- tests on pure & mixed state reconstruction and KL computation