Gpt1 for sequence classification (#7683)

* Add Documentation for GPT-1 Classification

* Add GPT-1 with Classification head

* Add tests for GPT-1 Classification

* Add GPT-1 For Classification to auto models

* Remove authorized missing keys, change checkpoint to openai-gpt

src/transformers/utils/dummy_pt_objects.py

@@ -1256,6 +1256,15 @@ class OpenAIGPTDoubleHeadsModel:
         requires_pytorch(self)
 
 
+class OpenAIGPTForSequenceClassification:
+    def __init__(self, *args, **kwargs):
+        requires_pytorch(self)
+
+    @classmethod
+    def from_pretrained(self, *args, **kwargs):
+        requires_pytorch(self)
+
+
 class OpenAIGPTLMHeadModel:
     def __init__(self, *args, **kwargs):
         requires_pytorch(self)