Xing Zhang

I haven't implemented the GPU backend for `eigh`. One workaround is to replace the `_eigh` function in the `hf.SCF` class by the one in https://github.com/fishjojo/pyscfad/blob/main/pyscfad/ml/scf/hf.py. But this may cause divergent...

It is doable, although some effort is needed to make it efficient. I may add the implicit differentiation for the Davidson solver, which should help excited state calculations.