update transformer XL with tf decorator
Author: infinite-Joy
Pull request: yes

# What does this PR do?

Unpacks TF model inputs through a decorator, improving code clarity for the transformer XL model. To be read with issue #16051 , which holds the description of the problem, the proposed solution, and future plan.

## 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?
- [x] Was this discussed/approved via a Github issue or the [forum](https://discuss.huggingface.co/)? Please add a link
      to it if that's the case. #16051 
- [ ] 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? @LysandreJik @gante 

Comments:
tests results using pytest 👇 

```
RUN_SLOW=1 pytest -vv tests/transfo_xl/test_modeling_tf_transfo_xl.py

tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_attention_outputs <- tests/test_modeling_tf_common.py PASSED                                   [  2%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_compile_tf_model <- tests/test_modeling_tf_common.py PASSED                                    [  5%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_config PASSED                                                                                  [  8%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_determinism <- tests/test_modeling_tf_common.py PASSED                                         [ 11%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_forward_signature <- tests/test_modeling_tf_common.py PASSED                                   [ 14%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_generate_with_headmasking <- tests/test_modeling_tf_common.py PASSED                           [ 17%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_headmasking <- tests/test_modeling_tf_common.py PASSED                                         [ 20%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_hidden_states_output <- tests/test_modeling_tf_common.py PASSED                                [ 23%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_initialization <- tests/test_modeling_tf_common.py PASSED                                      [ 26%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_inputs_embeds <- tests/test_modeling_tf_common.py PASSED                                       [ 29%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_keras_save_load <- tests/test_modeling_tf_common.py PASSED                                     [ 32%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_keyword_and_dict_args <- tests/test_modeling_tf_common.py PASSED                               [ 35%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_lm_head_model_beam_search_generate_dict_outputs <- tests/test_modeling_tf_common.py PASSED     [ 38%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_lm_head_model_no_beam_search_generate_dict_outputs <- tests/test_modeling_tf_common.py PASSED  [ 41%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_lm_head_model_random_beam_search_generate <- tests/test_modeling_tf_common.py PASSED           [ 44%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_lm_head_model_random_no_beam_search_generate <- tests/test_modeling_tf_common.py PASSED        [ 47%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_load_with_mismatched_shapes <- tests/test_modeling_tf_common.py PASSED                         [ 50%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_loss_computation <- tests/test_modeling_tf_common.py PASSED                                    [ 52%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_model_common_attributes PASSED                                                                 [ 55%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_model_from_pretrained PASSED                                                                   [ 58%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_model_main_input_name <- tests/test_modeling_tf_common.py PASSED                               [ 61%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_model_outputs_equivalence <- tests/test_modeling_tf_common.py PASSED                           [ 64%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_numpy_arrays_inputs <- tests/test_modeling_tf_common.py PASSED                                 [ 67%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_onnx_compliancy <- tests/test_modeling_tf_common.py PASSED                                     [ 70%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_onnx_runtime_optimize <- tests/test_modeling_tf_common.py PASSED                               [ 73%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_pt_tf_model_equivalence <- tests/test_modeling_tf_common.py SKIPPED (test is PT+TF test)       [ 76%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_resize_token_embeddings <- tests/test_modeling_tf_common.py PASSED                             [ 79%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_save_load <- tests/test_modeling_tf_common.py PASSED                                           [ 82%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_save_load_config <- tests/test_modeling_tf_common.py PASSED                                    [ 85%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_transfo_xl_lm_head PASSED                                                                      [ 88%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_transfo_xl_model PASSED                                                                        [ 91%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_transfo_xl_sequence_classification_model PASSED                                                [ 94%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelTest::test_xla_mode PASSED                                                                                [ 97%]
tests/transfo_xl/test_modeling_tf_transfo_xl.py::TFTransfoXLModelLanguageGenerationTest::test_lm_generate_transfo_xl_wt103 SKIPPED (Skip test until #12651 is resolved.)   [100%]
```

_The documentation is not available anymore as the PR was closed or merged._