Autoregressive and Relative Position Embedding Support

Hi! Nice work on the Longformer model! I am learning your model and have got some questions:

- I guess the code of LongformerSelfAttention has never enabled the `autoregressive` mode? Since I noticed that the `autoregressive` parameter is always set `False` when calling `diagonaled_mm_tvm`. If it is a bug, could you please fix it asap?
- Does this code support relative position embedding, as you mentioned in the paper that rpe is used in the autoregressive LM. If not, could you please release this part of code?

Thank you~