Optional layers (#8961)
* Apply on BERT and ALBERT
* Update TF Bart
* Add input processing to TF BART
* Add input processing for TF CTRL
* Add input processing to TF Distilbert
* Add input processing to TF DPR
* Add input processing to TF Electra
* Add deprecated arguments
* Add input processing to TF XLM
* remove unused imports
* Add input processing to TF Funnel
* Add input processing to TF GPT2
* Add input processing to TF Longformer
* Add input processing to TF Lxmert
* Apply style
* Add input processing to TF Mobilebert
* Add input processing to TF GPT
* Add input processing to TF Roberta
* Add input processing to TF T5
* Add input processing to TF TransfoXL
* Apply style
* Rebase on master
* Fix wrong model name
* Fix BART
* Apply style
* Put the deprecated warnings in the input processing function
* Remove the unused imports
* Raise an error when len(kwargs)>0
* test ModelOutput instead of TFBaseModelOutput
* Address Patrick's comments
* Address Patrick's comments
* Add boolean processing for the inputs
* Take into account the optional layers
* Add missing/unexpected weights in the other models
* Apply style
* rename parameters
* Apply style
* Remove useless
* Remove useless
* Remove useless
* Update num parameters
* Fix tests
* Address Patrick's comment
* Remove useless attribute
