Hi @ThaiThien,
Lets consider this model from https://github.com/leeyeehoo/CSRNet-pytorch 30
which has a forward function as following
def forward(self,x):
        x = self.frontend(x)
        x = self.backend(x)
        x = self.output_layer(x)
        return x

You can concatenate these three blocks (last one is a single layer for output) as below:
model_cat = torch.nn.Sequential(*model.frontend.children(),
                                *model.backend.children(),
                                model.output_layer)