Prepare to run next shot.

def prepare(self, cache):
        """Prepare to run next shot."""
        if cache is not None:
            np.copyto(self.qubits, cache)
        else:
            self.qubits.fill(0.0)
            self.qubits[0] = 1.0
        self.cregs = [0] * self.n_qubits