the master is defined in line 9 of the file project.conf
MASTER=
you enter the full path to the master (you should preprocess it manually, as explained by mdelgado here) and it should end with _Split_Orb.dim
The script “coreg_ifg_topsar.py” coregisters every slave product with the master.
Edit: Now I’m reading your question again and am no longer sure if you use the python scripts at all. I am not sure if the graph files of snap2stamps can be configured for batch use from within SNAP as well.