TF GPT2: clearer model variable naming with @unpack_inputs
# What does this PR do?
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Addresses https://github.com/huggingface/transformers/issues/16051


## Before submitting
- [ ] This PR fixes a typo or improves the docs (you can dismiss the other checks if that's the case).
- [x] Did you read the [contributor guideline](https://github.com/huggingface/transformers/blob/master/CONTRIBUTING.md#start-contributing-pull-requests),
      Pull Request section?
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- [x] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/master/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/master/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@gante @Rocketknight1 

Some tests failed locally: `12 failed, 37 passed, 1 skipped, 1 warning in 205.75s (0:03:25)`


One such test: `ValueError: The following keyword arguments are not supported by this model: ['past_key_values'].` even though GPT2 calls it `past` as opposed to `past_key_values`. Shouldn't the test be rewritten?<|||||>_The documentation is not available anymore as the PR was closed or merged._<|||||>Hey @cakiki 👋  The problem you're seeing is related to another change that is happening at the same time -- we are refactoring the auto-regressive generation function, where the `past` input variable in some old models got updated to `past_key_values`, to be consistent throughout models/frameworks.

(see next comment)<|||||>@sgugger @patrickvonplaten calling for your opinion here.

TL;DR:
- In a `generate()` refactor past PR, I made `prepare_inputs_for_generate()` uniform across famerworks. In the process, one of the output keys in TF GPT2 was updated from `past` to `past_key_values` -- removing the `past` was one of the TODO goals flagged by @patrickvonplaten;
- However, the current version of the model expects `past` as an input, if passed as a keyword argument [which raises the error @cakiki is seeing];
- In [PT](https://github.com/huggingface/transformers/blob/master/src/transformers/models/gpt2/modeling_gpt2.py#L1025)/[FLAX](https://github.com/huggingface/transformers/blob/master/src/transformers/models/gpt2/modeling_flax_gpt2.py#L456), this input is called `past_key_values`.

To fix it we have two options:
1. Revert the output of `prepare_inputs_for_generate()` from `past_key_values` to `past`;
2. We update the GPT2 input from `past` to `past_key_values`. It would be an API change, but this variable is mostly used in `generate()`, right?

I'm pro option 2, but WDYT?<|||||>I'm in favor of 1). I haven't done a good job at reviewing the refactor PR I think - see: https://github.com/huggingface/transformers/pull/15944#discussion_r832052267.

Backwards compatibility for models such as GPT2 is **extremely** important and people do use `past` outside of `generate`. Think the easy fix here is to just revert the line above. 

The other possibility is to deprecate `past` in general for all models in TF, but this should be done over a deprecation cycle so that users are aware of the change.<|||||>Were there other models where we renamed `past` to `past_key_values` without changing the keyword argument name in the forward function?<|||||>> Were there other models where we renamed `past` to `past_key_values` without changing the keyword argument name in the forward function?

Perhaps, going to check and open a PR to fix it (including this one)

@cakiki I will fix the issue in a separate PR, and will ping you to rebase with `main` when it is sorted<|||||>@cakiki the fix is merged -- rebasing with `main` should fix the problems you're seeing :)<|||||>(please rerun the tests locally before merging, and confirm here that they pass)<|||||>Rebasing with `main` did indeed solve most of the failing tests. The following 3 are still failing, but they're unrelated to the previous issue. 

```
======================================================================================================= short test summary info =======================================================================================================
FAILED tests/gpt2/test_modeling_tf_gpt2.py::TFGPT2ModelTest::test_gpt2_xla_generate - TypeError: function() got an unexpected keyword argument 'jit_compile'
FAILED tests/gpt2/test_modeling_tf_gpt2.py::TFGPT2ModelTest::test_onnx_runtime_optimize - ModuleNotFoundError: No module named 'onnxruntime'
FAILED tests/gpt2/test_modeling_tf_gpt2.py::TFGPT2ModelLanguageGenerationTest::test_lm_generate_gpt2_xla - TypeError: function() got an unexpected keyword argument 'jit_compile'
```<|||||>@cakiki 
- `jit_compile` as a flag of `tf.function()` was added in TF2.5, can you confirm that you have TF >= 2.5? If not, can you try reruning after updating TF to a version equal or higher than 2.5? [added a personal TODO to throw an error if TF<2.5]
- for the other error, can you try reruning the tests after reinstalling `transformers` with `pip install -e ".[dev,onnx]"`? It should be because of the onnx special dependencies :) 
<|||||>@gante I uninstalled tensorflow and explicitly pinned it >=2.5 and that worked. I noticed that `pip install .[dev]` was installing a bunch of tensorflow versions then finally settling on version 2.3 (fresh virtual env). (setup.py sets it to >=2.3)

![image](https://user-images.githubusercontent.com/3664563/159788909-fb2147b9-e93a-4097-9285-977380bfc966.png)
---
`test_onnx_runtime_optimize` is the only test still failing:
```
E           onnxruntime.capi.onnxruntime_pybind11_state.InvalidGraph: [ONNXRuntimeError] : 10 : INVALID_GRAPH : This is an invalid model. In Node, ("tfgp_t2for_sequence_classification_22/GatherV2", GatherV2, "", -1) : ("tfgp_t2for_sequence_classification_22/score/Tensordot:0": tensor(float),"tfgp_t2for_sequence_classification_22/sub:0": tensor(int32),"tfgp_t2for_sequence_classification_22/sub/y:0": tensor(int32),) -> ("logits": tensor(float),) , Error No Op registered for GatherV2 with domain_version of 10

../venv/lib/python3.6/site-packages/onnxruntime/capi/onnxruntime_inference_collection.py:370: InvalidGraph

```

```
ERROR    tf2onnx.tfonnx:tfonnx.py:303 Failed to convert node 'tfgp_t2for_sequence_classification_22/GatherV2' (fct=<bound method GatherV2.version_1 of <class 'tf2onnx.onnx_opset.tensor.GatherV2'>>)
'OP=GatherV2\nName=tfgp_t2for_sequence_classification_22/GatherV2\nInputs:\n\ttfgp_t2for_sequence_classification_22/score/Tensordot:0=Reshape, [-1, -1, 2], 1\n\ttfgp_t2for_sequence_classification_22/sub:0=Sub, [-1], 6\n\ttfgp_t2for_sequence_classification_22/GatherV2/axis:0=Const, [], 6\nOutpus:\n\ttfgp_t2for_sequence_classification_22/GatherV2:0=[-1, 2], 1'
Traceback (most recent call last):
  File "/media/ssd/BIGSCIENCE/venv/lib/python3.6/site-packages/tf2onnx/tfonnx.py", line 292, in tensorflow_onnx_mapping
    func(g, node, **kwargs, initialized_tables=initialized_tables, dequantize=dequantize)
  File "/media/ssd/BIGSCIENCE/venv/lib/python3.6/site-packages/tf2onnx/onnx_opset/tensor.py", line 444, in version_1
    utils.make_sure(node.get_attr_value("batch_dims", 0) == 0, err_msg)
  File "/media/ssd/BIGSCIENCE/venv/lib/python3.6/site-packages/tf2onnx/utils.py", line 260, in make_sure
    raise ValueError("make_sure failure: " + error_msg % args)
ValueError: make_sure failure: Opset 12 required for batch_dims attribute of GatherV2
```<|||||>If it helps:

```
- `transformers` version: 4.18.0.dev0
- Platform: Linux-4.15.0-171-generic-x86_64-with-Ubuntu-18.04-bionic
- Python version: 3.6.9
- Huggingface_hub version: 0.4.0
- PyTorch version (GPU?): 1.10.1+cu102 (True)
- Tensorflow version (GPU?): 2.6.2 (False)
- Flax version (CPU?/GPU?/TPU?): 0.3.5 (cpu)
- Jax version: 0.2.17
- JaxLib version: 0.1.69
```<|||||>@cakiki It is settling on TF 2.3 because of python 3.6-related limitations on several packages :( We are actually having an internal discussion about potentially dropping support to python 3.6, since it is causing issues with both TF and PT (e.g. TF 2.8 requires python >= 3.7).

As for the onnx test, let's not worry about it, since it is failing on master as well.<|||||>I see that there are further errors in the tests, I will take a look (I think I know how to fix it). I will ping here again when the related fix is merged.

Hah, it seems like you got the best model to apply `@unpack_inputs` on :D <|||||>@cakiki I've been working on issues related to TF GPT-2, and it seems like I've also solved the errors here. I've tested locally with these changes on top of `main`, and all tests pass (except the `onnx` one, which is also failing on `main`).

We've recently renamed our branch from `master` to `main`, so CI won't turn green until we rebase -- which I've just pushed :)<|||||>@gante Thank you for the update!<|||||>The failing test is being tracked internally -- merging

@cakiki thank you for the contribution! (and for the patience) 