Add DPR model (#5279)
* beginning of dpr modeling
* wip
* implement forward
* remove biencoder + better init weights
* export dpr model to embed model for nlp lib
* add new api
* remove old code
* make style
* fix dumb typo
* don't load bert weights
* docs
* docs
* style
* move the `k` parameter
* fix init_weights
* add pretrained configs
* minor
* update config names
* style
* better config
* style
* clean code based on PR comments
* change Dpr to DPR
* fix config
* switch encoder config to a dict
* style
* inheritance -> composition
* add messages in assert startements
* add dpr reader tokenizer
* one tokenizer per model
* fix base_model_prefix
* fix imports
* typo
* add convert script
* docs
* change tokenizers conf names
* style
* change tokenizers conf names
* minor
* minor
* fix wrong names
* minor
* remove unused convert functions
* rename convert script
* use return_tensors in tokenizers
* remove n_questions dim
* move generate logic to tokenizer
* style
* add docs
* docs
* quality
* docs
* add tests
* style
* add tokenization tests
* DPR full tests
* Stay true to the attention mask building
* update docs
* missing param in bert input docs
* docs
* style
Co-authored-by: Lysandre <lysandre.debut@reseau.eseo.fr>
