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

* fix gc bug

* continue PoC on OPT

* fixes

* 🤯

* fix tests

* remove pytest.mark

* fixup

* forward contrib credits from discussions

* forward contrib credits from discussions

* reverting changes on untouched files.

---------

Co-authored-by: zhaoqf123 <zhaoqf123@users.noreply.github.com>
Co-authored-by: 7eu7d7 <7eu7d7@users.noreply.github.com>

tests/models/speech_to_text/test_modeling_speech_to_text.py

@@ -324,6 +324,12 @@ class Speech2TextModelTest(ModelTesterMixin, GenerationTesterMixin, PipelineTest
     def test_training_gradient_checkpointing(self):
         pass
 
+    @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
+
     def test_generate_fp16(self):
         config, input_dict = self.model_tester.prepare_config_and_inputs()
         input_features = input_dict["input_features"]