[tutorial] Fix KeyError: 'w_old' (#1425)

Signed-off-by: Situ Singh <[email protected]>

Author: Situ Singh

openfl-tutorials/experimental/workflow/403_Federated_FedProx_PyTorch_MNIST_Workflow_Tutorial.ipynb

@@ -278,6 +278,8 @@
     "            output = self.model(data)\n",
     "            loss = F.cross_entropy(output, target)\n",
     "            loss.backward()\n",
+    "            \n",
+    "            self.optimizer.set_old_weights([p.clone().detach() for p in self.model.parameters()])\n",
     "            self.optimizer.step()\n",
     "\n",
     "            if (len(data) * batch_idx) / len(self.train_loader.dataset) >= log_threshold:\n",