Fix ODR violation from duplicate explicit instantiation of OperatorEXXPW

Move extern template declarations to op_pw_exx.h so all translation units
suppress implicit instantiation, and remove the duplicate explicit
instantiation in op_pw_exx_ace.cpp. The single explicit instantiation
in op_pw_exx.cpp remains the sole definition.

Author: abacus_fixer

source/source_pw/module_pwdft/op_pw_exx.h

@@ -175,6 +175,13 @@ class OperatorEXXPW : public OperatorPW<T, Device>
 
 };
 
+extern template class OperatorEXXPW<std::complex<float>, base_device::DEVICE_CPU>;
+extern template class OperatorEXXPW<std::complex<double>, base_device::DEVICE_CPU>;
+#if ((defined __CUDA) || (defined __ROCM))
+extern template class OperatorEXXPW<std::complex<float>, base_device::DEVICE_GPU>;
+extern template class OperatorEXXPW<std::complex<double>, base_device::DEVICE_GPU>;
+#endif
+
 template <typename Real, typename Device>
 void get_exx_potential(const K_Vectors* kv,
                        const ModulePW::PW_Basis_K* wfcpw,

source/source_pw/module_pwdft/op_pw_exx_ace.cpp

@@ -320,10 +320,4 @@ 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;
 }
-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