Add OPTForQuestionAnswering (#19402)

* Add `OPTForQuestionAnswering`

- added `OPTForQuestionAnswering` class based on `BloomForQuestionAnswering`
- added `OPTForQuestionAnswering` in common tests
- all common tests pass
- make fixup done

* added docstrings for OPTForQuestionAnswering

* Fix docstrings for OPTForQuestionAnswering

tests/models/opt/test_modeling_opt.py

@@ -32,7 +32,13 @@ from ...test_modeling_common import ModelTesterMixin, ids_tensor
 if is_torch_available():
     import torch
 
-    from transformers import GPT2Tokenizer, OPTForCausalLM, OPTForSequenceClassification, OPTModel
+    from transformers import (
+        GPT2Tokenizer,
+        OPTForCausalLM,
+        OPTForQuestionAnswering,
+        OPTForSequenceClassification,
+        OPTModel,
+    )
 
 
 def prepare_opt_inputs_dict(
@@ -178,7 +184,11 @@ class OPTModelTester:
 
 @require_torch
 class OPTModelTest(ModelTesterMixin, GenerationTesterMixin, unittest.TestCase):
-    all_model_classes = (OPTModel, OPTForCausalLM, OPTForSequenceClassification) if is_torch_available() else ()
+    all_model_classes = (
+        (OPTModel, OPTForCausalLM, OPTForSequenceClassification, OPTForQuestionAnswering)
+        if is_torch_available()
+        else ()
+    )
     all_generative_model_classes = (OPTForCausalLM,) if is_torch_available() else ()
     is_encoder_decoder = False
     fx_compatible = True