Add TAPEX (#16473)

* Add TapexTokenizer

* Improve docstrings and provide option to provide answer

* Remove option for pretokenized inputs

* Add TAPEX to README

* Fix copies

* Remove option for pretokenized inputs

* Initial commit: add tapex fine-tuning examples on both table-based question answering and table-based fact verification.

* - Draft a README file for running the script and introducing some background.
- Remove unused code lines in tabfact script.
- Disable the deafult `pad_to_max_length` option which is memory-consuming.

* * Support `as_target_tokenizer` function for TapexTokenizer.
* Fix the do_lower_case behaviour of TapexTokenizer.
* Add unit tests for target scenarios and cased/uncased scenarios for both source and target.

* * Replace the label BartTokenizer with TapexTokenizer's as_target_tokenizer function.
* Fix typos in tapex example README.

* * fix the evaluation script - remove the property `task_name`

* * Make the label space more clear for tabfact tasks

* * Using a new fine-tuning script for tapex-base on tabfact.

* * Remove the lowercase code outside the tokenizer - we use the tokenizer to control whether do_lower_case
* Guarantee the hyper-parameter can be run without out-of-memory on 16GB card and report the new reproduced number on wikisql

* * Remove the default tokenizer_name option.
* Provide evaluation command.

* * Support for WikiTableQuestion dataset.

* Fix a typo in README.

* * Fix the datasets's key name in WikiTableQuestions

* Run make fixup and move test to folder

* Fix quality

* Apply suggestions from code review

* Apply suggestions from code review
Co-authored-by: Suraj Patil <surajp815@gmail.com>

* Apply suggestions from code review

* Apply suggestions from code review
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* Apply some more suggestions from code review

* Improve docstrings

* Overwrite failing test

* Improve comment in example scripts

* Fix rebase

* Add TAPEX to Auto mapping

* Add TAPEX to auto config mappings

* Put TAPEX higher than BART in auto mapping

* Add TAPEX to doc tests
Co-authored-by: Niels Rogge <nielsrogge@Nielss-MBP.localdomain>
Co-authored-by: SivilTaram <qianlxc@outlook.com>
Co-authored-by: Niels Rogge <nielsrogge@nielss-mbp.home>
Co-authored-by: Suraj Patil <surajp815@gmail.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Niels Rogge <nielsrogge@Nielss-MacBook-Pro.local>

utils/documentation_tests.txt

 docs/source/en/quicktour.mdx
 docs/source/en/task_summary.mdx
 docs/source/en/model_doc/speech_to_text.mdx
+docs/source/en/model_doc/tapex.mdx
 src/transformers/generation_utils.py
 src/transformers/models/bart/modeling_bart.py
 src/transformers/models/beit/modeling_beit.py