[BUG] to_uhf, to_uks, and other conversion methods are NotImplemented in RHF/RKS

[wtpeter]

The conversion methods (e.g., to_uhf, to_uks, to_rks, etc.) in gpu4pyscf.scf.hf.RHF and related classes are currently marked as NotImplemented.

In gpu4pyscf/scf/hf.py, these methods are set to:

    to_rhf                   = NotImplemented
    to_uhf                   = NotImplemented
    to_ghf                   = NotImplemented
    to_rks                   = NotImplemented
    to_uks                   = NotImplemented
    # ... and so on

A simple wrapper leveraging the existing to_cpu() and to_gpu() methods could bridge this gap:

def to_uhf(self):
    return self.to_cpu().to_uhf().to_gpu()