Confusion Matrix - BWT, FWT metrics

Hello,

How can I have access to the confusion matrix per experience? I have two experiences.

I have this integrated the code below, but I dont know how to access confusion matrix.

```
eval_plugin = 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),
bwt_metrics(experience=True, stream=True),
forward_transfer_metrics(experience=True, stream=True),
forgetting_metrics(experience=True, stream=True),
cpu_usage_metrics(experience=True),
confusion_matrix_metrics(num_classes=len(benchmark.train_stream[0].classes_in_this_experience), save_image=True, stream=True),
disk_usage_metrics(minibatch=True, epoch=True, experience=True, stream=True),
loggers=[InteractiveLogger()],
strict_checks=False
)
```

In the end, I see the following:
`ConfusionMatrix_Stream/eval_phase/test_stream = `

Also, when I use as metrics Forward Knowledge Transfer and Backward Knowledge Transfer, I get this error:

Thank you.