Hi,
self.sgcn2 is sequential of MS_GCN, MS_TCN, and MS_TCN.
So, self.sgcn2[-1] refers to the last MS_TCN.
self.sgcn2[-1].act is either the attribute or member variable of an instance of MS_TCN. From that name, I guess by default MS_TCN has some activation function such as Sigmoid and ReLU. Though the author of that snippet wanted that last module to use Identity as its activation.