[T5] Add T5ForQuestionAnswering and MT5ForQuestionAnswering (#24481)

* Adding T5ForQuestionAnswering

* Changed weight initialization that results in better initial loss when fine-tuning

* Update to class variables

* Running make fixup

* Running make fix-copies

* Remove model_parallel

* Adding MT5ForQuestionAnswering

* Adding docs

* Fix wrong doc

* Update src/transformers/models/mt5/modeling_mt5.py

Co-authored-by: Younes Belkada <49240599+younesbelkada@users.noreply.github.com>

* Update src/transformers/models/t5/modeling_t5.py

Co-authored-by: Younes Belkada <49240599+younesbelkada@users.noreply.github.com>

* File formatting

* Undoing change

---------

Co-authored-by: Younes Belkada <49240599+younesbelkada@users.noreply.github.com>

tests/models/t5/test_modeling_t5.py

@@ -43,6 +43,7 @@ if is_torch_available():
         ByT5Tokenizer,
         T5EncoderModel,
         T5ForConditionalGeneration,
+        T5ForQuestionAnswering,
         T5Model,
         T5Tokenizer,
     )
@@ -520,7 +521,7 @@ class T5ModelTester:
 
 @require_torch
 class T5ModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMixin, unittest.TestCase):
-    all_model_classes = (T5Model, T5ForConditionalGeneration) if is_torch_available() else ()
+    all_model_classes = (T5Model, T5ForConditionalGeneration, T5ForQuestionAnswering) if is_torch_available() else ()
     all_generative_model_classes = (T5ForConditionalGeneration,) if is_torch_available() else ()
     pipeline_model_mapping = (
         {
@@ -529,6 +530,7 @@ class T5ModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMixin,
             "summarization": T5ForConditionalGeneration,
             "text2text-generation": T5ForConditionalGeneration,
             "translation": T5ForConditionalGeneration,
+            "question-answering": T5ForQuestionAnswering,
         }
         if is_torch_available()
         else {}