Generation tests: update imagegpt input name, remove unused functions (#33663)

tests/models/mamba/test_modeling_mamba.py

@@ -143,25 +143,6 @@ class MambaModelTester:
         config.vocab_size = 300
         return config
 
-    def prepare_config_and_inputs_for_decoder(self):
-        (
-            config,
-            input_ids,
-            attention_mask,
-            sequence_labels,
-            token_labels,
-            choice_labels,
-        ) = self.prepare_config_and_inputs()
-
-        return (
-            config,
-            input_ids,
-            attention_mask,
-            sequence_labels,
-            token_labels,
-            choice_labels,
-        )
-
     def create_and_check_mamba_model(self, config, input_ids, *args):
         config.output_hidden_states = True
         model = MambaModel(config=config)