Generate: make TF `.generate()` signature == PT `.generate()` signature 
# What does this PR do?

TF generation test addition PR 3 (out of ???).

In an effort to move generation integration tests to be framework-agnostic, I'll be adding low-hanging fruit to TF. This PR adds framework-agnostic tests for the signature of `.generate()`, which means that the TF signature had to be updated 🙃 

Please note that this change is backward compatible -- regardless of whether the user was passing `.generate(input_ids)`, `.generate(input_ids=input_ids)`, or (e.g.) `.generate(pixel_values=pixel_values)`, no change on their end is needed.
_The documentation is not available anymore as the PR was closed or merged._