New TF model inputs
# What does this PR do?

This PR improves the way the inputs are handled in the TensorFlow models:

- It should be easier now to write a TensorFlow model
- The order of the inputs are now better handled, mostly when using Keras Tensors
- Replace all the occurrences of `inputs` by `input_ids` to make easier to understand what this parameter is for and to align with PyTorch input names and the Tokenizers outputs.

@LysandreJik @sgugger @patrickvonplaten let me know what you think about this new input processing, it is not finished yet but any comment will be helpful for me :)
@sgugger Thanks for these useful comments, I should have addressed them as you proposed. All the modification have been directly integrated in the new `input_processing` function. can you please also check the `modeling_tf_bart.py`, I have updated the documentation, let me know if I did something wrong.

@patrickvonplaten can you please check the same BART file as I have done some updates in order to make it able to properly handle the `return_dict=True`  and `return_dict=False` my updates don't affect the usual behaviour of the model even if the tests are ok.<|||||>@LysandreJik @sgugger @patrickvonplaten Do you think it would be interesting to have the same thing for the outputs? knowing that for graph mode compliance we will need to have such or such output behaviour depending the state we are (eager or not). <|||||>> Remember to do separate PRs for separate issues please, a lot of changes in TFBart here are unrelated to the main focus of the PR.

Do you prefer that I move the Bart changes into another PR and keep only the inputs changes? I don't mind to do this :)<|||||>I have also updated the TF template.<|||||>> Do you prefer that I move the Bart changes into another PR and keep only the inputs changes? I don't mind to do this :)

Since @patrickvonplaten approved, I think they're okay here for this time (unless he says otherwise ;-) ) <|||||>I would prefer the same, it looks a bit too big to be added at this point of the release. I should have addressed all the comments. Let me know if I missed some.