Cannot use checkpoint.WeightCheckpoint in evaluation metrics

🐛 **The Bug**
Was trying to save model weights using checkpoint evaluation metric, but failed with the following error

```python
TypeError Traceback (most recent call last)
in ()
10
11 print('Computing accuracy on the whole test set')
---> 12 results.append(cl_strategy.eval(scenario.test_stream))

6 frames
/usr/lib/python3.7/copy.py in deepcopy(x, memo, _nil)
167 reductor = getattr(x, "__reduce_ex__", None)
168 if reductor:
--> 169 rv = reductor(4)
170 else:
171 reductor = getattr(x, "__reduce__", None)

TypeError: can't pickle generator objects
```

🐜 **To Reproduce**
Follow the tutorial [evaluation page](https://avalanche.continualai.org/from-zero-to-hero-tutorial/05_evaluation)
```python
EvaluationPlugin(
accuracy_metrics(minibatch=True, epoch=True, experience=True, stream=True),
loss_metrics(minibatch=True, epoch=True, experience=True, stream=True),
timing_metrics(epoch=True, epoch_running=True),
forgetting_metrics(experience=True, stream=True),
forward_transfer_metrics(experience=True, stream=True),
confusion_matrix_metrics(
num_classes=scenario.n_classes,
save_image=True,
stream=True
),
[checkpoint.WeightCheckpoint()],
loggers=[interactive_logger, text_logger, tb_logger]
)
```
Then train the model normally, the weight failed to be saved

🐝 **Expected behavior**
Guessing that the model weight would be saved somewhere, but unsure how to use it properly