Fix DARTS 2nd order (#4385)

nni/retiarii/oneshot/pytorch/darts.py

@@ -214,7 +214,7 @@ class DartsTrainer(BaseOneShotTrainer):
         # calculate unrolled loss on validation data
         # keep gradients for model here for compute hessian
         _, loss = self._logits_and_loss(val_X, val_y)
-        w_model, w_ctrl = tuple(self.model.parameters()), tuple([c.alpha for c in self.nas_modules])
+        w_model, w_ctrl = tuple(self.model.parameters()), tuple([c.alpha for _, c in self.nas_modules])
         w_grads = torch.autograd.grad(loss, w_model + w_ctrl)
         d_model, d_ctrl = w_grads[:len(w_model)], w_grads[len(w_model):]
 
@@ -267,7 +267,7 @@ class DartsTrainer(BaseOneShotTrainer):
                     p += e * d
 
             _, loss = self._logits_and_loss(trn_X, trn_y)
-            dalphas.append(torch.autograd.grad(loss, [c.alpha for c in self.nas_modules]))
+            dalphas.append(torch.autograd.grad(loss, [c.alpha for _, c in self.nas_modules]))
 
         dalpha_pos, dalpha_neg = dalphas  # dalpha { L_trn(w+) }, # dalpha { L_trn(w-) }
         hessian = [(p - n) / (2. * eps) for p, n in zip(dalpha_pos, dalpha_neg)]