Unnecessarily strict requirement for batch size in MASPlugin

**Describe the bug**

In the MASPlugin, the following check seems to be unnecessarily strict:

[Link to code](https://github.com/ContinualAI/avalanche/blob/afebe34d3f927a0ecefc11a884c7b0282899bc2e/avalanche/training/plugins/mas.py#L84)
```python
for _, batch in enumerate(dataloader):
# Get batch
if len(batch) == 2 or len(batch) == 3:
x, _, t = batch[0], batch[1], batch[-1]
else:
raise ValueError("Batch size is not valid")
```

As a result, if I have a custom dataset which returns additional elements (two images and a label), thi

**Expected behavior**

I am not sure MAS needs to follow this check, it can simply grab the needed information (image, label, and token) and continue onwards.