Ok here is what was happening.
This notation in CLI script did not work
python preprocessing_script.py
--Y_df ${{outputs.Y_df}}
--S_df ${{outputs.S_df}}
Thats because hydra does not like that notation (I think)
Instead this notation worked:
python data_processing.py '+Y_df=${{outputs.Y_df}}' '+S_df=${{outputs.S_df}}'
What this does is that it adds those 2 new variables - Y_df and S_df into the config file
These variables can be accessed in the program just like all other variables in the config file by doing cfg.Y_df
or cfg.S_df