Irreasonable time consumption in cal_pdm() in DeePKS.

[ErjieWu]

Details

When the number of k points raise, the running time increases notebly and becomes unacceptable. This may caused by DMR calculation in cal_pdm() function in DeePKS.

It seems some of steps in calculate_HR() in DeePKS can also be reduced.

[ErjieWu]

It seems this issue has already be solved in #6094, which update the way for DMR saving and calculation.