transformers

56ee2560 - [Longformer] Better handling of global attention mask vs local attention mask (#4672)

[Longformer] Better handling of global attention mask vs local attention mask (#4672) * better api * improve automatic setting of global attention mask * fix longformer bug * fix global attention mask in test * fix global attn mask flatten * fix slow tests * update docstring * update docs and make more robust * improve attention mask