[WIP] Add Tapas (bis)
This is a clean branch based on #8113 which is up-to-date with master. To do:

- [x] Make sure all tests pass (currently 44 passed, 4 skipped for `test_modeling_tapas.py`) cc @LysandreJik 
- [x] `make style` & `make quality`
- [x] Investigating forward/backward pass => weird issue with fine-tuning already-finetuned WTQ checkpoint, I guess people should just not do it
- [x]  Add notebooks to show how to use: 
- `tapas-base-finetuned-sqa`: https://colab.research.google.com/drive/1zMW-D2kYrpDA-cvpNJ-ctGD-tDXWebZa?usp=sharing
- `tapas-base-finetuned-tabfact`: https://colab.research.google.com/drive/1Ug6gzPFgf3J0dR-0f4spt0eyPS10dD1l?usp=sharing

Once they all pass, I'll start uploading more checkpoints to the model hub.

Closing this PR and opening a new one on the same branch due to Github issues.