Longformer for question answering (#4500)
* added LongformerForQuestionAnswering
* add LongformerForQuestionAnswering
* fix import for LongformerForMaskedLM
* add LongformerForQuestionAnswering
* hardcoded sep_token_id
* compute attention_mask if not provided
* combine global_attention_mask with attention_mask when provided
* update example in docstring
* add assert error messages, better attention combine
* add test for longformerForQuestionAnswering
* typo
* cast gloabl_attention_mask to long
* make style
* Update src/transformers/configuration_longformer.py
* Update src/transformers/configuration_longformer.py
* fix the code quality
* Merge branch 'longformer-for-question-answering' of https://github.com/patil-suraj/transformers into longformer-for-question-answering
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>
