Add support for ModernBertForMultipleChoice (#39232)

* implement ModernBertForMultipleChoice

* fixup, style, repo consistency

* generate modeling_modernbert

* add tests + docs

* fix test

tests/models/modernbert/test_modeling_modernbert.py

@@ -41,6 +41,7 @@ if is_torch_available():
     from transformers import (
         MODEL_FOR_PRETRAINING_MAPPING,
         ModernBertForMaskedLM,
+        ModernBertForMultipleChoice,
         ModernBertForQuestionAnswering,
         ModernBertForSequenceClassification,
         ModernBertForTokenClassification,
@@ -202,6 +203,22 @@ class ModernBertModelTester:
         result = model(input_ids, attention_mask=input_mask, labels=token_labels)
         self.parent.assertEqual(result.logits.shape, (self.batch_size, self.seq_length, self.num_labels))
 
+    def create_and_check_for_multiple_choice(
+        self, config, input_ids, input_mask, sequence_labels, token_labels, choice_labels
+    ):
+        config.num_labels = self.num_labels
+        model = ModernBertForMultipleChoice(config=config)
+        model.to(torch_device)
+        model.eval()
+        multiple_choice_inputs_ids = input_ids.unsqueeze(1).expand(-1, self.num_choices, -1).contiguous()
+        multiple_choice_input_mask = input_mask.unsqueeze(1).expand(-1, self.num_choices, -1).contiguous()
+        result = model(
+            multiple_choice_inputs_ids,
+            attention_mask=multiple_choice_input_mask,
+            labels=choice_labels,
+        )
+        self.parent.assertEqual(result.logits.shape, (self.batch_size, self.num_choices))
+
     def prepare_config_and_inputs_for_common(self):
         config_and_inputs = self.prepare_config_and_inputs()
         (
@@ -227,6 +244,7 @@ class ModernBertModelTest(ModelTesterMixin, PipelineTesterMixin, unittest.TestCa
             ModernBertForSequenceClassification,
             ModernBertForTokenClassification,
             ModernBertForQuestionAnswering,
+            ModernBertForMultipleChoice,
         )
         if is_torch_available()
         else ()
@@ -298,6 +316,7 @@ class ModernBertModelTest(ModelTesterMixin, PipelineTesterMixin, unittest.TestCa
                         ModernBertForSequenceClassification,
                         ModernBertForTokenClassification,
                         ModernBertForQuestionAnswering,
+                        ModernBertForMultipleChoice,
                     ]
                 ):
                     self.assertIn(
@@ -318,6 +337,10 @@ class ModernBertModelTest(ModelTesterMixin, PipelineTesterMixin, unittest.TestCa
         config_and_inputs = self.model_tester.prepare_config_and_inputs()
         self.model_tester.create_and_check_for_token_classification(*config_and_inputs)
 
+    def test_for_multiple_choice(self):
+        config_and_inputs = self.model_tester.prepare_config_and_inputs()
+        self.model_tester.create_and_check_for_multiple_choice(*config_and_inputs)
+
     def test_for_warning_if_padding_and_no_attention_mask(self):
         (
             config,