Add BloomForQuestionAnswering (#19310)

* add bloom for question answering

- attempt to add Bloom for question answering
- adapted from `GPTJForQuestionAnswering`
- Fixed `num_labels` to `2` for common tests
- Added a bit of docstring
- All common tests pass

* Update src/transformers/models/bloom/modeling_bloom.py

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

* revert changes related to `num_labels`

Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>

docs/source/en/model_doc/bloom.mdx

@@ -55,3 +55,8 @@ Several smaller versions of the models have been trained on the same dataset. BL
 
 [[autodoc]] BloomForTokenClassification
     - forward
+
+## BloomForQuestionAnswering
+
+[[autodoc]] BloomForQuestionAnswering
+    - forward