Fix: use correct \psi for GPU in cal_tau for ELF (#7378)

* Fix: use correct PSI for GPU in cal_tau for ELF

* Refactor: split long cast into readable lines in after_scf

Author: Flying-dragon-boxing

source/source_esolver/esolver_ks_pw.cpp

@@ -279,7 +279,9 @@ void ESolver_KS_PW<T, Device>::after_scf(UnitCell& ucell, const int istep, const
     // Calculate kinetic energy density tau for ELF if needed
     if (PARAM.inp.out_elf[0] > 0)
     {
-        this->pelec->cal_tau(*(this->stp.psi_cpu));
+        auto* elec_pw = static_cast<elecstate::ElecStatePW<T, Device>*>(this->pelec);
+        auto& psi = *this->stp.template get_psi_t<T, Device>();
+        elec_pw->cal_tau(psi);
     }
 
     ESolver_KS::after_scf(ucell, istep, conv_esolver);