Chen Nuo

### Background #5463 When using `DiagH_subspace` in cg method, we construct an overlap matrix `scc` by default. This can be a huge burden on memory. Initial `psi` set is not...

Unnecessary overhead for solving eigenvalues in diago_dav_subspace

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### Details `hegvd` is called to solve all the eigenvalues where only some of them are required, i.e. `hegvx` should be called instead. See https://github.com/deepmodeling/abacus-develop/blob/6f277a75a36c87b42ccb4bedf0d044bfb132767e/source/source_hsolver/diago_dav_subspace.cpp#L543 --- Full code: ```cpp if...