Xin, please send these messages to the Lammps user list.
The Two Temperature Model (TTM) has two independent temperature fields: one for phonons and one for electrons. When you apply a flux, you must specify which field it is being applied to, otherwise it would be ambiguous where you wanted the energy.
We use the standard definition for a flux: energy per area per time. You have correctly identified the ATC units base you should use. Note however you are specifying a source, not a flux with the command. A source is a volumetric energy source, i.e., energy per volume per time.