Revert "Fix gradient checkpointing + fp16 autocast for most models" (#24420)

Revert "Fix gradient checkpointing + fp16 autocast for most models (#24247)"

This reverts commit 285a48011d.

tests/models/gpt2/test_modeling_gpt2.py

@@ -562,12 +562,6 @@ class GPT2ModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTesterMixin
         config_and_inputs = self.model_tester.prepare_config_and_inputs()
         self.model_tester.create_and_check_gpt2_weight_initialization(*config_and_inputs)
 
-    @unittest.skip(
-        reason="The model does not support GC + autocast + fp16: https://github.com/huggingface/transformers/pull/24247"
-    )
-    def test_training_gradient_checkpointing_autocast(self):
-        pass
-
     @slow
     def test_batch_generation(self):
         model = GPT2LMHeadModel.from_pretrained("gpt2")