fix(exx): add explicit template instantiation in op_pw_exx_ace.cpp

op_pw_exx_ace.cpp defines ACE-specific template members
(act_op_ace, construct_ace, cal_exx_energy_ace) but lacked explicit
instantiations. The extern template class declaration in op_pw_exx.h
suppressed implicit instantiation everywhere, and the template class
instantiations in op_pw_exx.cpp could not see definitions in this
translation unit, leading to linker undefined reference errors.

Add template class instantiations for CPU and GPU variants at the end
of op_pw_exx_ace.cpp so these members emit symbols.

Author: abacus_fixer

source/source_pw/module_pwdft/op_pw_exx_ace.cpp

@@ -320,4 +320,13 @@ double OperatorEXXPW<T, Device>::cal_exx_energy_ace(psi::Psi<T, Device>* ppsi_)
     Eexx = Eexx / hybrid_alpha / 2; // This factor of 2 is from the definition of EXX energy.
     return Eexx;
 }
+
+// Explicit instantiation for members defined in this translation unit.
+template class OperatorEXXPW<std::complex<float>, base_device::DEVICE_CPU>;
+template class OperatorEXXPW<std::complex<double>, base_device::DEVICE_CPU>;
+#if ((defined __CUDA) || (defined __ROCM))
+template class OperatorEXXPW<std::complex<float>, base_device::DEVICE_GPU>;
+template class OperatorEXXPW<std::complex<double>, base_device::DEVICE_GPU>;
+#endif
+
 } // namespace hamilt