Add multi-class, multi-label and regression to transformers (#11012)

* add to  bert

* review comments

* Update src/transformers/configuration_utils.py

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

* Update src/transformers/configuration_utils.py

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

* self.config.problem_type

* fix style

* fix

* fin

* fix

* update doc

* fix

* test

* Test more problem types

* Update src/transformers/configuration_utils.py

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

* fix

* remove

* fix

* quality

* make fix-copies

* remove test

Co-authored-by: abhishek thakur <abhishekkrthakur@users.noreply.github.com>
Co-authored-by: Sylvain Gugger <35901082+sgugger@users.noreply.github.com>
Co-authored-by: Lysandre <lysandre.debut@reseau.eseo.fr>

tests/test_modeling_convbert.py

@@ -260,6 +260,7 @@ class ConvBertModelTest(ModelTesterMixin, unittest.TestCase):
     )
     test_pruning = False
     test_head_masking = False
+    test_sequence_classification_problem_types = True
 
     def setUp(self):
         self.model_tester = ConvBertModelTester(self)