Adapting this repo to current version of transformers library

Hi,

First of all, thank all of you for the amazing work!
I was wondering if there is any plan to update this repo to work with the latest version of HF transformers library.

When I try to load the model I got this kind of error (similar errors are reported in other issues):

```
RuntimeError: Error(s) in loading state_dict for LongformerEncoderDecoderForConditionalGeneration:
size mismatch for model.encoder.embed_positions.weight: copying a param with shape torch.Size([4098, 1024]) from checkpoint, the shape in current model is torch.Size([1026, 1024]).
```

Is there an effective way to make it work?