TF: unpack inputs on Convbert, GPTJ, LED, and templates
# What does this PR do?

This PR implements the `@unpack_inputs` decorator on Convbert, GPTJ, LED, and templates, as per https://github.com/huggingface/transformers/issues/16051.

The diff is big, but the changes are all trivial:
1. Replace `input_processing()` calls by the `@unpack_inputs` decorator
2. Replace `inputs["foo"]` by `foo` (thank god regex exists 🙏 )
3. (SLOW tests were run locally, all passing)

Only one PR remains to close the issue linked above -- implement the change in the encoder_decoder architectures. I've decided to create a separate PR for that one as the changes are not as trivial, and will require a review from a core maintainer.
_The documentation is not available anymore as the PR was closed or merged._