[`MBart`] Fix cross attention mask check
# What does this PR do?

Fixes https://github.com/huggingface/transformers/issues/21728

The current `MBart` code leads to an error that is hard to interpret for users due to a possible typo as pointed out in https://github.com/huggingface/transformers/issues/21728 
To reproduce: 
```python
import torch
from transformers import MBartModel

input_ids = torch.LongTensor([[0, 1, 1, 0]])
model = MBartModel.from_pretrained("facebook/mbart-large-cc25")

head_mask = None

cross_attn_head_mask = torch.ones(1000, 1, 1, 1)
model(input_ids, head_mask=head_mask, cross_attn_head_mask=cross_attn_head_mask)
```

This PR fixes this

cc @sgugger 
_The documentation is not available anymore as the PR was closed or merged._