Fix for the TauGNN implementation

This MR fixes a small bug in the TauGNN class, where the m_config object was left uninitialized, resulting in the track and cluster GNN variables going into the same temporary vector during the graph construction. Therefore, any track variable with the same name as a cluster variable used by the network was being overwritten with the cluster's values. This wasn't noticed before because in the Offline TauID, no two variables with the same name were used.

Tagging @ntamir.