Add support for pretraining recurring span selection to Splinter (#17247)

* Add SplinterForSpanSelection for pre-training recurring span selection.

* Formatting.

* Rename SplinterForSpanSelection to SplinterForPreTraining.

* Ensure repo consistency

* Fixup changes

* Address SplinterForPreTraining PR comments

* Incorporate feedback and derive multiple question tokens per example.

* Update src/transformers/models/splinter/modeling_splinter.py

Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

* Update src/transformers/models/splinter/modeling_splinter.py

Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

Co-authored-by: Jean Vancoppenole <jean.vancoppenolle@retresco.de>
Co-authored-by: Tobias Günther <tobias.guenther@retresco.de>
Co-authored-by: Tobias Günther <github@tobigue.de>
Co-authored-by: Patrick von Platen <patrick.v.platen@gmail.com>

docs/source/en/model_doc/splinter.mdx

@@ -72,3 +72,8 @@ This model was contributed by [yuvalkirstain](https://huggingface.co/yuvalkirsta
 
 [[autodoc]] SplinterForQuestionAnswering
     - forward
+
+## SplinterForPreTraining
+
+[[autodoc]] SplinterForPreTraining
+    - forward