Removed inputs_processing and replaced with decorator for lxmert
Author: silvererudite
Pull request: yes

# What does this PR do?
Replaces usage of `inputs_processing` with decorator `unpack_input` in `lxmert`


See #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/main/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.
- [ ] Did you make sure to update the documentation with your changes? Here are the
      [documentation guidelines](https://github.com/huggingface/transformers/tree/main/docs), and
      [here are tips on formatting docstrings](https://github.com/huggingface/transformers/tree/main/docs#writing-source-documentation).
- [ ] Did you write any new necessary tests?


## Who can review?

@gante 

Comments:
All tests pass after running `RUN_SLOW=1 py.test -vv tests/lxmert/test_modeling_tf_lxmert.py`
```
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_attention_outputs PASSED                                                                                                       [  2%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_compile_tf_model PASSED                                                                                                        [  5%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_config PASSED                                                                                                                  [  8%] 
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_determinism <- tests\test_modeling_tf_common.py PASSED                                                                         [ 11%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_forward_signature <- tests\test_modeling_tf_common.py PASSED                                                                   [ 14%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_generate_with_headmasking <- tests\test_modeling_tf_common.py PASSED                                                           [ 17%] 
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_headmasking <- tests\test_modeling_tf_common.py PASSED                                                                         [ 20%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_hidden_states_output PASSED                                                                                                    [ 23%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_initialization <- tests\test_modeling_tf_common.py PASSED                                                                      [ 26%] 
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_inputs_embeds <- tests\test_modeling_tf_common.py PASSED                                                                       [ 29%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_keras_save_load <- tests\test_modeling_tf_common.py PASSED                                                                     [ 32%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_keyword_and_dict_args <- tests\test_modeling_tf_common.py PASSED                                                               [ 35%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_lm_head_model_beam_search_generate_dict_outputs <- tests\test_modeling_tf_common.py PASSED                                     [ 38%] 
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_lm_head_model_no_beam_search_generate_dict_outputs <- tests\test_modeling_tf_common.py PASSED                                  [ 41%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_lm_head_model_random_beam_search_generate <- tests\test_modeling_tf_common.py PASSED                                           [ 44%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_lm_head_model_random_no_beam_search_generate <- tests\test_modeling_tf_common.py PASSED                                        [ 47%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_load_with_mismatched_shapes <- tests\test_modeling_tf_common.py PASSED                                                         [ 50%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_loss_computation <- tests\test_modeling_tf_common.py PASSED                                                                    [ 52%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_lxmert_for_pretraining PASSED                                                                                                  [ 55%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_lxmert_model PASSED                                                                                                            [ 58%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_model_common_attributes PASSED                                                                                                 [ 61%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_model_from_pretrained SKIPPED (test is slow)                                                                                   [ 64%] 
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_model_main_input_name <- tests\test_modeling_tf_common.py PASSED                                                               [ 67%] 
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_model_outputs_equivalence <- tests\test_modeling_tf_common.py PASSED                                                           [ 70%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_numpy_arrays_inputs <- tests\test_modeling_tf_common.py PASSED                                                                 [ 73%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_onnx_compliancy <- tests\test_modeling_tf_common.py PASSED                                                                     [ 76%] 
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_onnx_runtime_optimize <- tests\test_modeling_tf_common.py SKIPPED (test is slow)                                               [ 79%] 
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_pt_tf_model_equivalence PASSED                                                                                                 [ 82%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_resize_token_embeddings <- tests\test_modeling_tf_common.py PASSED                                                             [ 85%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_save_load PASSED                                                                                                               [ 88%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_save_load_config <- tests\test_modeling_tf_common.py PASSED                                                                    [ 91%]
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_saved_model_creation PASSED                                                                                                    [ 94%] 
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelTest::test_saved_model_creation_extended SKIPPED (test is slow)                                                                           [ 97%] 
tests/lxmert/test_modeling_tf_lxmert.py::TFLxmertModelIntegrationTest::test_inference_masked_lm SKIPPED (test is slow)                                                                          [100%] 
```

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

Hi again @gante remade all the changes in this fresh PR..so sorry that you need to review once again. In turn I realized I need to learn more `git` 😅