Feature Request: Support Longformer 3D attention mask ?
I try to use 3d attntion mask, but failed.
Hey @Maybewuss,
could you post a code snippet example, where you show the use case of using a 3D attention mask? :-) 
What would you use it for?<|||||>> Hey @Maybewuss,
> could you post a code snippet example, where you show the use case of using a 3D attention mask? :-)
> What would you use it for?

@patrickvonplaten 
I want each word to foucs different words in an sentence, for example, in the generate task, the attntion mask is a lower triangular 3d matrix. Besides, other models like Bert and Roberta support 3d mask.

```
model = Lonformer.from_pretrained('model_path')
input_ids = torch.randint(20, (3, 5), dtype=int)
attn_masks = torch.randint(2, (3, 5, 5), dtype=int)
model(input_ids, attn_masks)
```<|||||>@Maybewuss , I think this feature should actually already be implemented for Longformer.

Can you provide me with a code snippet with the error message?
The code snippet should include a public model and functional code so that I can copy-paste into my console to see the error. Otherwise, it will be difficult for me to help you. Thanks! <|||||>This issue has been automatically marked as stale because it has not had recent activity. It will be closed if no further activity occurs. Thank you for your contributions.
<|||||>I used 3d attention mask in the LongFormer, but also failed. I find that 
the code 
attention_mask = nn.functional.pad(
                attention_mask, (0, padding_len), value=0
            )  # no attention on the padding tokens 
in line 1626 in modeling_longformer may be not support the 3D attention mask. Please correct me if I am wrong